UserStore migration (#15563)

* Migration completed

* Fix tests

* Fix tests

* Fix tests

* Suggestions

* Trigger CI

* Suggestions

* Merge with master

* Trigger CI

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>

cmd/mattermost/commands/userargs.go

@@ -21,7 +21,7 @@ func getUserFromUserArg(a *app.App, userArg string) *model.User {
 	user, _ := a.Srv().Store.User().GetByEmail(userArg)
 
 	if user == nil {
-		var err *model.AppError
+		var err error
 		if user, err = a.Srv().Store.User().GetByUsername(userArg); err == nil {
 			return user
 		}