adapt replication-backend to give access only to own user's notifications_xxx DB

[sleidig]

• Check and possible adapt the Permission system in the replication-backend as we introduce additional databases (notifications-<USER_ID>). Do the endpoints still function as expected?
• Adapt the permission system to grant/deny access to a whole user-specific database:
  • e.g. by default a user should always access the CouchDBs for that account (notifications-<MY_USER_ID>) but never those of other accounts (notifications-<OTHER_USER_ID>).
  • an explicit permission for the NotificationEvent entity type shouldn't be required in Config:Permissions (because the access is handled by splitting it into the user-specific couchdb)
  • How do we encode or hard-code these rules?
  • Implement the additional checks

[sleidig]

The sync with additional CouchDB's (other than "app" also syncing "notifications-xxx") seems to work fine without explicit changes to replication-backend