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

app/notification_test.go

@@ -41,7 +41,7 @@ func TestSendNotifications(t *testing.T) {
 		t.Fatal("user should have been mentioned")
 	}
 
-	dm, err := th.App.CreateDirectChannel(th.BasicUser.Id, th.BasicUser2.Id)
+	dm, err := th.App.GetOrCreateDirectChannel(th.BasicUser.Id, th.BasicUser2.Id)
 	if err != nil {
 		t.Fatal(err)
 	}