[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>

model/client4.go

@@ -3100,6 +3100,24 @@ func (c *Client4) GetChannelsForTeamAndUserWithLastDeleteAt(teamId, userId strin
 	return ch, BuildResponse(r), nil
 }
 
+// GetChannelsForUserWithLastDeleteAt returns a list channels for a user, additionally filtered with lastDeleteAt.
+func (c *Client4) GetChannelsForUserWithLastDeleteAt(userID string, lastDeleteAt int) ([]*Channel, *Response, error) {
+	route := fmt.Sprintf(c.userRoute(userID) + "/channels")
+	route += fmt.Sprintf("?last_delete_at=%d", lastDeleteAt)
+	r, err := c.DoAPIGet(route, "")
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+
+	var ch []*Channel
+	err = json.NewDecoder(r.Body).Decode(&ch)
+	if err != nil {
+		return nil, BuildResponse(r), NewAppError("GetChannelsForUserWithLastDeleteAt", "api.marshal_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+	return ch, BuildResponse(r), nil
+}
+
 // SearchChannels returns the channels on a team matching the provided search term.
 func (c *Client4) SearchChannels(teamId string, search *ChannelSearch) ([]*Channel, *Response, error) {
 	searchJSON, jsonErr := json.Marshal(search)
@@ -3160,6 +3178,29 @@ func (c *Client4) SearchAllChannels(search *ChannelSearch) (ChannelListWithTeamD
 	return ch, BuildResponse(r), nil
 }
 
+// SearchAllChannelsForUser search in all the channels for a regular user.
+func (c *Client4) SearchAllChannelsForUser(term string) (ChannelListWithTeamData, *Response, error) {
+	search := &ChannelSearch{
+		Term: term,
+	}
+	searchJSON, jsonErr := json.Marshal(search)
+	if jsonErr != nil {
+		return nil, nil, NewAppError("SearchAllChannelsForUser", "api.marshal_error", nil, jsonErr.Error(), http.StatusInternalServerError)
+	}
+	r, err := c.DoAPIPost(c.channelsRoute()+"/search?system_console=false", string(searchJSON))
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+
+	var ch ChannelListWithTeamData
+	err = json.NewDecoder(r.Body).Decode(&ch)
+	if err != nil {
+		return nil, BuildResponse(r), NewAppError("SearchAllChannelsForUser", "api.marshal_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+	return ch, BuildResponse(r), nil
+}
+
 // SearchAllChannelsPaged searches all the channels and returns the results paged with the total count.
 func (c *Client4) SearchAllChannelsPaged(search *ChannelSearch) (*ChannelsWithCount, *Response, error) {
 	searchJSON, jsonErr := json.Marshal(search)
@@ -3304,7 +3345,7 @@ func (c *Client4) GetChannelByNameForTeamNameIncludeDeleted(channelName, teamNam
 	return ch, BuildResponse(r), nil
 }
 
-// GetChannelMembers gets a page of channel members.
+// GetChannelMembers gets a page of channel members specific to a channel.
 func (c *Client4) GetChannelMembers(channelId string, page, perPage int, etag string) (ChannelMembers, *Response, error) {
 	query := fmt.Sprintf("?page=%v&per_page=%v", page, perPage)
 	r, err := c.DoAPIGet(c.channelMembersRoute(channelId)+query, etag)
@@ -3321,6 +3362,23 @@ func (c *Client4) GetChannelMembers(channelId string, page, perPage int, etag st
 	return ch, BuildResponse(r), nil
 }
 
+// GetChannelMembersWithTeamData gets a page of all channel members for a user.
+func (c *Client4) GetChannelMembersWithTeamData(userID string, page, perPage int) (ChannelMembersWithTeamData, *Response, error) {
+	query := fmt.Sprintf("?page=%v&per_page=%v", page, perPage)
+	r, err := c.DoAPIGet(c.userRoute(userID)+"/channel_members"+query, "")
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+
+	var ch ChannelMembersWithTeamData
+	err = json.NewDecoder(r.Body).Decode(&ch)
+	if err != nil {
+		return nil, BuildResponse(r), NewAppError("GetChannelMembersWithTeamData", "api.marshal_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+	return ch, BuildResponse(r), nil
+}
+
 // GetChannelMembersByIds gets the channel members in a channel for a list of user ids.
 func (c *Client4) GetChannelMembersByIds(channelId string, userIds []string) (ChannelMembers, *Response, error) {
 	r, err := c.DoAPIPost(c.channelMembersRoute(channelId)+"/ids", ArrayToJSON(userIds))