fix against https://github.com/globaleaks/GlobaLeaks/issues/498
fix against https://github.com/globaleaks/GlobaLeaks/issues/483
aligned an UT in test_authentication.py to latest implemementation
implemented fields validation fixes https://github.com/globaleaks/GlobaLeaks/issues/518
removed a test reintroduced by an erroneous merge
added possibility to resfresh session by doing a GET on the /authentication API