* Removal of ldapalchemy and LDAP mocking support
* Removal of dependency on ldap3 (except for the migration)
* Remaining "LDAP_<name>" config keys are renamed to "<name>"
* Web interface to create, edit and delete groups
* Consistent foreign key, cascading and nullable configuration on all models
* User/Group.dn is replaced with numeric User/Group.id
* User.uid is renamed to User.unix_uid (to differentiate with new User.id)
* Group.gid is renamed to Group.unix_gid (to differentiate with new Group.id)
* All User/Group/Mail related routes now use the database ids instead of uid/gid/dn
* PasswordToken/MailToken now reference users directly instead of storing loginnames

The database migration (optionally) uses the v1 config keys to connect to
an LDAP server and to import all users, groups and mail forwardings.

flask_oauthlib is no longer available in Debian Bullseye. It is only a
wrapper around oauthlib, which is still available. While this change does
increase the OAuth2 code size, it achieves compatability with both Debian
Buster and Bullseye.

Aside from error handling, this change has no noticable effects on OAuth2.0
clients. In terms of error handling, a few cases that were not properly
handled before now return appropriate error pages.

Fixes #101