Migrate OAuth2 and API clients to database

Removes support of 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.

TODOs:

• Add confirmation for delete button
• Decide on client_id naming confusion
• Integrate service_admin module in services module
• Unit tests
• Add useful migration that migrates client definitions from the config to the database
• Manual testing
• Document changes in update instructions
• Update translations

Closes #107 (closed)