lib: drop own asbool implementation and consistently use tg.support.converters as utils2.asbool

str2bool never reported error on odd input such as '' or '-1', but the tg
asbool behaviour of raising ValueError("String is not true/false: %r" % obj) in
that case seems fine.

py3: remove safe_unicode in places where it no longer is needed because all strings (except bytes) already *are* unicode strings

(The remaining safe_unicode calls are still needed and can't just be removed,
generally because we in these cases still have to convert from bytes to unicode
strings.)

auth: move 'active' handling out of the individual auth modules

The 'active' flag in the Kallithea user database is very fundamental and should
not be specific to auth modules. Modules should only care about whether the
user is active in the external authentication system.

user_activation_state is thus removed, and 'hg.extern_activate.auto' is now
consistently checked for all kinds of external authentication.

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.

auth: get username as str when container auth gets username from user object

Avoid stray unicode username.

Correct licensing information in individual files.

The top-level license file is now LICENSE.md.

Also, in various places where there should have been joint copyright holders
listed, a single copyright holder was listed. It does not appear easy to add
a link to a large list of copyright holders in these places, so it simply
refers to the fact that various authors hold copyright.

In future, if an easy method is discovered to link to a list from those
places, we should do so.

Finally, text is added to LICENSE.md to point to where the full list of
copyright holders is, and that Kallithea as a whole is GPLv3'd.

Second step in two-part process to rename directories.
This is the actual directory rename.