Migrate User.Save to Sync by default (#11482)

* Migrate User.Save to Sync by default

* Make error testing more explicit

* Factor out else

* Fix some merge-related bugs

* Add some missing nil err test requirements

* Improve function signature

* Remove unused time import

* More test fixups

app/user.go

@@ -293,12 +293,11 @@ func (a *App) createUser(user *model.User) (*model.User, *model.AppError) {
 		return nil, err
 	}
 
-	result := <-a.Srv.Store.User().Save(user)
-	if result.Err != nil {
-		mlog.Error(fmt.Sprintf("Couldn't save the user err=%v", result.Err))
-		return nil, result.Err
+	ruser, err := a.Srv.Store.User().Save(user)
+	if err != nil {
+		mlog.Error(fmt.Sprintf("Couldn't save the user err=%v", err))
+		return nil, err
 	}
-	ruser := result.Data.(*model.User)
 
 	if user.EmailVerified {
 		if err := a.VerifyUserEmail(ruser.Id, user.Email); err != nil {