fix: change auth:login to fall back to cookie auth

[wolfgangmm]

Restore the old behaviour of auth:login for backwards compatibility with TP 10 and earlier:

• fall back to cookie auth if no user given
• support use of parameter logout to log out

[DrRataplan]

this adds code that will be hopefully soon be dead code: when pb-login does the auth check in a different way.

A unit test would be nice here.

[line-o]

@DrRataplan Yes, I also think we need to ensure the functionality for the next releases we need to keep this in.

I can offer to create the test if @wolfgangmm does not have the time.