db: mark migration as completed when it's done

migrated_to_db config key is used. Might also add an extra check to see
if anything is in the DB.

migrations.go

@@ -280,6 +280,11 @@ func migrateToBadger(app *appContext) {
 	if _, ok := app.storage.GetCustomContentKey("UserPage"); !ok {
 		app.storage.SetCustomContentKey("UserPage", app.storage.deprecatedUserPageContent.Page)
 	}
+
+	tempConfig, _ := ini.Load(app.configPath)
+	tempConfig.Section("").Key("migrated_to_db").SetValue("true")
+	tempConfig.SaveTo(app.configPath)
+	app.info.Println("All data migrated to database. JSON files in the config folder can be deleted if you are sure all data is correct in the app. Create an issue if you have problems.")
 }
 
 // Migrate between hyphenated & non-hyphenated user IDs. Doesn't seem to happen anymore, so disabled.