Cleanup and separate controllers

This MR cleans up the controller code and form validation. It also separates the Audience controllers in order to have shorter controllers that are better maintainable.

In this MR, the new AudienceParticipantController is still quite long, but that will be fixed as soon as #8 (closed) is implemented.

This branch includes the following fixes:

• AudienceEditController has been split up into AudienceEditController, AudienceDocumentController and AudienceParticipantController. This fixes #39 (closed).
• The participant POST endpoints do not longer live on /organizer/audience/{id}/*-participant but rather on /organizer/audience/{id}/participant/*, which is more in line with the new AudienceParticipantController.
• The HTML index pages have been updated to by default show the buttons that can be used without JavaScript. This is especially useful in an environment where the JavaScript is loading slow.
• .editorconfig has been updated with an IntelliJ specific option to have a greater indent for line continuations. Otherwise, it would conflict with our checkstyle configuration.
• The tests have been updated such that they reflect the new changes.