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.

Fixes #114

Regression was introduced by 45d4598e (Replace flask_oauthlib with plain
oauthlib). Support for HTTP Basic authentication scheme for client
authentication is actually required by RFC6749.

Fixes #115

Regression was introduced by 45d4598e (Replace flask_oauthlib with plain oauthlib).

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

Add a few unit tests as well as integration tests that cover almost all views. Also fixed a lot of HTML validator errors.