Tests

Add a few unit tests as well as integration tests that cover almost all views.

Webauthn (mfa) setup/verification is not covered as it is quite difficult to test (python-fido2 does not implement an authenticator for testing). Also ldap utils and user/group/role/mail models are not covered by unit tests (but should be!). The test cases for role and mail views are very basic and should be extended.

Also fixed a lot of HTML validator errors.