[MM-18946] [MM-26721] [MM-6842] Cross team search+private channel autocomplete (#18468)

* Show private channels in autocomplete

This is supported in all Engines:
MySQL, Postgres, Bleve, Elasticsearch.

https://mattermost.atlassian.net/browse/MM-18496

```release-note
Private channels will now appear in channel autocomplete.
If you are using Bleve or ElasticSearch, you will have to reindex
the channels again to populate them with the new attributes.
```

A large chunk of this work has been based on the earlier
effort at https://github.com/mattermost/mattermost-server/pull/17804.

Full credit goes to https://github.com/arvinDarmawan.

* Add comment

```release-note
NONE
```

* Adding more tests

```release-note
NONE
```

* fix more tests

```release-note
NONE
```

* tmp

```release-note
NONE
```

* more fixes

```release-note
NONE
```

* add tests

```release-note
NONE
```

* Add review comments from previous PR

```release-note
NONE
```

* Add API to return all channels from all team

```release-note
NONE
```

* Added support for bleve and ES

```release-note
NONE
```

* Streaming response for GetAllChannels

```release-note
NONE
```

* Fix tests

```release-note
NONE
```

* Trigger CI
```release-note
NONE
```

* fix tests

```release-note
NONE
```

* Addressing review comments

```release-note
NONE
```

* Fix lint

```release-note
NONE
```

* Removing flaky test

```release-note
NONE
```

* Address comments

```release-note
NONE
```

* Trigger CI
```release-note
NONE
```

* Added /users/<userid>/channel_members endpoint

```release-note
NONE
```

* Minor edit

```release-note
NONE
```

* Improve embedding

```release-note
NONE
```

* Fix lint error

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/channel_test.go

@@ -1034,6 +1034,58 @@ func TestGetChannelsForTeamForUser(t *testing.T) {
 	})
 }
 
+func TestGetChannelsForUser(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	client := th.Client
+
+	// Adding another team with more channels (public and private)
+	myTeam := th.CreateTeam()
+	ch1 := th.CreateChannelWithClientAndTeam(client, model.ChannelTypeOpen, myTeam.Id)
+	ch2 := th.CreateChannelWithClientAndTeam(client, model.ChannelTypePrivate, myTeam.Id)
+	th.LinkUserToTeam(th.BasicUser, myTeam)
+	th.App.AddUserToChannel(th.BasicUser, ch1, false)
+	th.App.AddUserToChannel(th.BasicUser, ch2, false)
+
+	channels, _, err := client.GetChannelsForUserWithLastDeleteAt(th.BasicUser.Id, 0)
+	require.NoError(t, err)
+
+	numPrivate := 0
+	numPublic := 0
+	numOffTopic := 0
+	numTownSquare := 0
+	for _, ch := range channels {
+		if ch.Type == model.ChannelTypeOpen {
+			numPublic++
+		} else if ch.Type == model.ChannelTypePrivate {
+			numPrivate++
+		}
+
+		if ch.DisplayName == "Off-Topic" {
+			numOffTopic++
+		} else if ch.DisplayName == "Town Square" {
+			numTownSquare++
+		}
+	}
+
+	assert.Len(t, channels, 9)
+	assert.Equal(t, 2, numPrivate)
+	assert.Equal(t, 7, numPublic)
+	assert.Equal(t, 2, numOffTopic)
+	assert.Equal(t, 2, numTownSquare)
+
+	// Creating some more channels to be exactly 100 to test page size boundaries.
+	for i := 0; i < 91; i++ {
+		ch1 = th.CreateChannelWithClientAndTeam(client, model.ChannelTypeOpen, myTeam.Id)
+		th.App.AddUserToChannel(th.BasicUser, ch1, false)
+	}
+
+	channels, _, err = client.GetChannelsForUserWithLastDeleteAt(th.BasicUser.Id, 0)
+	require.NoError(t, err)
+	assert.Len(t, channels, 100)
+}
+
 func TestGetAllChannels(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
@@ -1383,6 +1435,16 @@ func TestSearchAllChannels(t *testing.T) {
 	require.NoError(t, err)
 
 	team := th.CreateTeam()
+	privateChannel2, _, err := th.SystemAdminClient.CreateChannel(&model.Channel{
+		DisplayName: "dn_private2",
+		Name:        "private2",
+		Type:        model.ChannelTypePrivate,
+		TeamId:      team.Id,
+	})
+	require.NoError(t, err)
+	th.LinkUserToTeam(th.SystemAdminUser, team)
+	th.LinkUserToTeam(th.SystemAdminUser, th.BasicTeam)
+
 	groupConstrainedChannel, _, err := th.SystemAdminClient.CreateChannel(&model.Channel{
 		DisplayName:      "SearchAllChannels-groupConstrained-1",
 		Name:             "groupconstrained1",
@@ -1450,7 +1512,7 @@ func TestSearchAllChannels(t *testing.T) {
 		{
 			"Search with private channel filter",
 			&model.ChannelSearch{Private: true},
-			[]string{th.BasicPrivateChannel.Id, th.BasicPrivateChannel2.Id, privateChannel.Id, groupConstrainedChannel.Id},
+			[]string{th.BasicPrivateChannel.Id, privateChannel2.Id, th.BasicPrivateChannel2.Id, privateChannel.Id, groupConstrainedChannel.Id},
 		},
 		{
 			"Search with public channel filter",
@@ -1517,6 +1579,14 @@ func TestSearchAllChannels(t *testing.T) {
 		})
 	}
 
+	userChannels, _, err := th.SystemAdminClient.SearchAllChannelsForUser("private")
+	require.NoError(t, err)
+	assert.Len(t, userChannels, 2)
+
+	userChannels, _, err = th.SystemAdminClient.SearchAllChannelsForUser("FOOBARDISPLAYNAME")
+	require.NoError(t, err)
+	assert.Len(t, userChannels, 1)
+
 	// Searching with no terms returns all default channels
 	allChannels, _, err := th.SystemAdminClient.SearchAllChannels(&model.ChannelSearch{Term: ""})
 	require.NoError(t, err)
@@ -3238,7 +3308,7 @@ func TestAutocompleteChannels(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 
-	// A private channel to make sure private channels are not used
+	// A private channel to make sure private channels are used.
 	ptown, _, _ := th.Client.CreateChannel(&model.Channel{
 		DisplayName: "Town",
 		Name:        "town",
@@ -3267,8 +3337,8 @@ func TestAutocompleteChannels(t *testing.T) {
 			"Basic town-square",
 			th.BasicTeam.Id,
 			"town",
-			[]string{"town-square"},
-			[]string{"off-topic", "town", "tower"},
+			[]string{"town-square", "town"},
+			[]string{"off-topic", "tower"},
 		},
 		{
 			"Basic off-topic",
@@ -3281,8 +3351,8 @@ func TestAutocompleteChannels(t *testing.T) {
 			"Basic town square and off topic",
 			th.BasicTeam.Id,
 			"tow",
-			[]string{"town-square", "tower"},
-			[]string{"off-topic", "town"},
+			[]string{"town-square", "tower", "town"},
+			[]string{"off-topic"},
 		},
 	} {
 		t.Run(tc.description, func(t *testing.T) {