auth: drop active_from_extern from internal auth API

Modules should never auth a user if the auth source knows the user is inactive.
Also, it is too late and unreliable to disable users when they try to log in.
There is thus no need for this concept.

Only the crowd module had some traces of actual active_from_extern usage. The
'active' flag for crowd users was fully controlled from crowd. Now, Instead,
just let crowd reject authentication of users that are inactive in crowd, and
leave the internal Kallithea 'active' flag under admin control.