MM-23015: Enable or disable group mentions (#14010)

* MM-23015: Enable or disable group mentions + show them in suggestion list


Co-authored-by: Catalin Tomai <catalin.tomai@mattermost.com>

app/app_iface.go

@@ -517,6 +517,7 @@ type AppIface interface {
 	GetGroupSyncable(groupID string, syncableID string, syncableType model.GroupSyncableType) (*model.GroupSyncable, *model.AppError)
 	GetGroupSyncables(groupID string, syncableType model.GroupSyncableType) ([]*model.GroupSyncable, *model.AppError)
 	GetGroups(page, perPage int, opts model.GroupSearchOpts) ([]*model.Group, *model.AppError)
+	GetGroupsAssociatedToChannelsByTeam(teamId string, opts model.GroupSearchOpts) (map[string][]*model.GroupWithSchemeAdmin, *model.AppError)
 	GetGroupsByChannel(channelId string, opts model.GroupSearchOpts) ([]*model.GroupWithSchemeAdmin, int, *model.AppError)
 	GetGroupsByIDs(groupIDs []string) ([]*model.Group, *model.AppError)
 	GetGroupsBySource(groupSource model.GroupSource) ([]*model.Group, *model.AppError)