addressed issue https://github.com/globaleaks/GlobaLeaks/issues/1007

@hellais i'm going to open a twin pull for this on glbackend.

the current glclient implementation (in general) has some limits:
1) authentication is not really handled in a structured/layered way where each action is controlled by the glclient befor being performed. but all is before performed and then checked for errors (e.g. login redirect appens after a user tries to accesse an authenticated resource and the backed deny the access)
2) the same as 1) will need to be applied for authentication.statuses (here with this commit i'm introducing the first set: enabled, disabled, password_change_needed.