style: make arrows in usergroup members selector easier to press

bugfix: santize wouldnt allow files starting with . - fixes #3936

authn: Fix handling of form errors and default values.

tests: Adapt test to changed template.

templates: Add an extension point to the authn plugin settings template. Allows derived templates to add something below the form fileds.

authn: Set the renderer attribute when registering settings view.

authn: Adapt and refactor plugin settings template. Adapted to the refactoring of settings views.

authn: Remove unused code Unused due to the previoud refactoring.

authn: Refactored the auth-plugins-settings base view. This allows plugins to easier extend the template context dict without duplicate the code of the base views.

pull-requests: make the catch all route also support nicer format with '-'