Honor user.primaryGroupID in addition to memberOf.
Also, the group choice dialog now offers all groups anyone can have.

/close #160

Also fixes some latent, rather unimportant bugs

Only define authentication backends that require external packages if
their required modules have been found.
There is no need to install packages for unused backends.

ref #160

/close #144

ref #116

And show a warning if a user logs in with one of them.

This adds default values to default metadata and the option to set it
before compiling the protocol (i.e. while planning to TO).
Additionally, the (dynamic) metadata fields are handled correctly in the
ProtocolForm and metadata is shown while still planning.
Additionally, protocol.start_time is used instead of
protocoltype.usual_time.

/close #47
/close #65
/close #106

Enable using multiple authentication backends.
(Samba-)AD and LDAP-Backends are implemented.
Session timeouts are implemented.

/close #17