MM-45723: Include DM/GM channels for user indexing (#21391)

There were 2 separate issues here:

1. We were excluding DM/GM channels for user indexing.
Not sure why. This prevented users to be shown as
channel members in autocomplete results in both
Bleve and Elasticsearch.

2. We were incorrectly deciding to search across
the team based on if the current channel
is in the list of allowed channels or not. But we
were incorrectly wiping the list of allowed channels
even if a channelid was passed. This makes it work
same as a DB search.

One catch is that for the correct results to appear,
a re-indexing is required.

```release-note
Autocompletion results using Elasticsearch or Bleve will
correctly show a user as a channel member in DM/GM channels.
To force this change to appear, a re-indexing will be
necessary.
```

store/storetest/user_store.go

@@ -4912,10 +4912,35 @@ func testUserStoreGetUsersBatchForIndexing(t *testing.T, ss store.Store) {
 	})
 	require.NoError(t, err)
 
+	cDM := &model.Channel{
+		Name: model.NewId() + "__" + model.NewId(),
+		Type: model.ChannelTypeDirect,
+	}
+	cm1 := &model.ChannelMember{
+		UserId:      u3.Id,
+		ChannelId:   cDM.Id,
+		NotifyProps: model.GetDefaultChannelNotifyProps(),
+	}
+	cm2 := &model.ChannelMember{
+		UserId:      u2.Id,
+		ChannelId:   cDM.Id,
+		NotifyProps: model.GetDefaultChannelNotifyProps(),
+	}
+	cDM, nErr = ss.Channel().SaveDirectChannel(cDM, cm1, cm2)
+	require.NoError(t, nErr)
+
 	// Getting all users
 	res1List, err := ss.User().GetUsersBatchForIndexing(u1.CreateAt-1, "", 100)
 	require.NoError(t, err)
 	assert.Len(t, res1List, 3)
+	for _, user := range res1List {
+		switch user.Id {
+		case u2.Id:
+			assert.ElementsMatch(t, user.ChannelsIds, []string{cPub1.Id, cPub2.Id, cDM.Id})
+		case u3.Id:
+			assert.ElementsMatch(t, user.ChannelsIds, []string{cPub2.Id, cDM.Id})
+		}
+	}
 
 	// Testing pagination
 	res2List, err := ss.User().GetUsersBatchForIndexing(u1.CreateAt-1, "", 1)