Remove LDAP support

Fixes #73 (closed)

Todos:

• Adapt LDAP-to-DB migration to recent changes and get it working with MySQL
• Fix remaining TODOs in LDAP-to-DB migration
• Replace CLI command "create-examples" with commands to create users, groups and roles -> #117 (closed)
• Add/adapt documentation
• Tests for the new group views
• Full review of all models
  • Ensure that nullable is correct
  • Ensure that foreign keys are correct
  • Ensure that cascades are correct
• Compare schema export from SQLite and MySQL resulting from flask db upgrade and db.create_all()