[MM-7968] restrict creation of direct channels to team members (#17222)

* restrict creation of direct channels to team members

* run make i18n-extract

* add suggestions from hahmadia

* place common-team-check logic in app layer

* use flat SQL query

* show more specific error message to user

* MM-7968: Fmt file.

* MM-7968: Fix for moved session field.

Co-authored-by: Max Erenberg <max.erenberg@mattermost.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Martin Kraft <martinkraft@gmail.com>
Co-authored-by: Martin Kraft <martin@upspin.org>

store/store.go

@@ -150,6 +150,10 @@ type TeamStore interface {
 
 	// GroupSyncedTeamCount returns the count of non-deleted group-constrained teams.
 	GroupSyncedTeamCount() (int64, error)
+
+	// GetCommonTeamIDsForTwoUsers returns the intersection of all the teams to which the specified
+	// users belong.
+	GetCommonTeamIDsForTwoUsers(userID, otherUserID string) ([]string, error)
 }
 
 type ChannelStore interface {