Add config option 'groups_admin' to allow users to become dokuwiki admins by being in a list of django groups

Fix: Don't show logout link if logoff URI is not set

CRITICAL FIX: Don't give every Django user with an email address full admin privileges in the dokuwiki

Cleanup, fix constructor parent calling

Make all django users member of dokuwiki group 'user', add options to make django admins and/or staff members of dokuwiki group 'admin'

Add support for new session data encoding since Django 4, automatically decoding old and new data

Add new config option to add a logoff URI

Remove outdated description comment

Use getConf instead of directly accessing the config dict

Add config description strings

Add plugin code