[IMP] Use pretty urls for category modification
[IMP] Allow RHs to call e.g. another RH cleanly

Uses ResponseUtil to ensure the callable is not called until the
original RH finished and e.g. closed the DB connection
[IMP] Clean URLs for event home page and creation

Also a bunch of improvements/refactorings:
- shorturl routing
- url shows "lecture" instead of "simple_event" for lecture creation
[IMP] Shorturl handling

new namespaces: /c/xxx and /e/xxx
old ones are still supported (but not displayed anywhere)
shorturls are validated properly