Rebuild database task (#900)

* Improved tests for multi-user mode
* Adds task to rebuild database
* Updated subscriptions.js export syntax
* Subscription metadata is now backed up
* Added typing to task key
* Updated api models
* Tasks actions styling update

backend/consts.js

@@ -347,6 +347,8 @@ const YTDL_ARGS_WITH_VALUES = [
     '--convert-subs'
 ];
 
+exports.SUBSCRIPTION_BACKUP_PATH = 'subscription_backup.json'
+
 // we're using a Set here for performance
 exports.YTDL_ARGS_WITH_VALUES = new Set(YTDL_ARGS_WITH_VALUES);