implemented backend support for https://github.com/globaleaks/GlobaLeaks/issues/953 and https://github.com/globaleaks/GlobaLeaks/issues/936
added db for testing of further migrations starting from version 14
Merge branch 'feature/936and953' into devel

* feature/936and953:
  added db for testing of further migrations starting from version 14
  implemented backend support for https://github.com/globaleaks/GlobaLeaks/issues/953 and https://github.com/globaleaks/GlobaLeaks/issues/936