implemented feature to disable private messages
implemented https://github.com/globaleaks/GlobaLeaks/issues/953
minor fix needed for https://github.com/globaleaks/GlobaLeaks/issues/951
Merge remote-tracking branch 'origin/feature/disable_private_messages' into feature/936and953

* origin/feature/disable_private_messages:
  implemented feature to disable private messages
implemented MVP for https://github.com/globaleaks/GlobaLeaks/issues/950
removed backup file