MM-10121: CLI command to reset permissions system to default state. (#8637)

* MM-10121: CLI command to reset permissions system to default state.

* Review comment.

app/permissions.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2018-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func (a *App) ResetPermissionsSystem() *model.AppError {
+	// Purge all roles from the database.
+	if result := <-a.Srv.Store.Role().PermanentDeleteAll(); result.Err != nil {
+		return result.Err
+	}
+
+	// Remove the "System" table entry that marks the advanced permissions migration as done.
+	if result := <-a.Srv.Store.System().PermanentDeleteByName(ADVANCED_PERMISSIONS_MIGRATION_KEY); result.Err != nil {
+		return result.Err
+	}
+
+	// Now that the permissions system has been reset, re-run the migration to reinitialise it.
+	a.DoAdvancedPermissionsMigration()
+
+	return nil
+}