Also adds a shallow Service model that coexists with the config-defined
services to group multiple OAuth2 and API clients together.

Clients defined in the config with OAUTH2_CLIENTS and API_CLIENTS_2 are
imported by the database migrations.

Removes support for complex values for the OAuth2 client group_required option.
Only simple group names are supported, not (nested) lists of groups previously
interpreted as AND/OR conjunctions. Also removes support for the login_message
parameter of OAuth2 clients.

* 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.