MM-9728: Online migration for advanced permissions phase 2 (#8744)

* MM-9728: Online migration for advanced permissions phase 2

* Add unit tests for new store functions.

* Move migration specific code to own file.

* Add migration state function test.

* Style fixes.

* Add i18n strings.

* Fix mocks.

* Add TestMain to migrations package tests.

* Fix typo.

* Fix review comments.

* Fix up the "Check if migration is done" check to actually work.

app/scheme.go

@@ -3,7 +3,9 @@
 
 package app
 
-import "github.com/mattermost/mattermost-server/model"
+import (
+	"github.com/mattermost/mattermost-server/model"
+)
 
 func (a *App) GetScheme(id string) (*model.Scheme, *model.AppError) {
 	if result := <-a.Srv.Store.Scheme().Get(id); result.Err != nil {
@@ -109,7 +111,9 @@ func (a *App) GetChannelsForScheme(scheme *model.Scheme, offset int, limit int)
 }
 
 func (a *App) IsPhase2MigrationCompleted() *model.AppError {
-	// TODO: Actually check the Phase 2 migration has completed before permitting these actions.
+	if result := <-a.Srv.Store.System().GetByName(model.MIGRATION_KEY_ADVANCED_PERMISSIONS_PHASE_2); result.Err != nil {
+		return result.Err
+	}
 
 	return nil
 }