Refactor Get/Create Direct Channel into one function (#9867)

* refactor GetDirectChannel and CreateDirectChannel in one function

* remove CreateDirectChannel plugin api and update GetDirectChannel and GetGroupChannel plugin api

* update tests

migrations/migrationstestlib.go

@@ -227,7 +227,7 @@ func (me *TestHelper) CreateDmChannel(user *model.User) *model.Channel {
 	utils.DisableDebugLogForTest()
 	var err *model.AppError
 	var channel *model.Channel
-	if channel, err = me.App.CreateDirectChannel(me.BasicUser.Id, user.Id); err != nil {
+	if channel, err = me.App.GetOrCreateDirectChannel(me.BasicUser.Id, user.Id); err != nil {
 		mlog.Error(err.Error())
 
 		time.Sleep(time.Second)