Move trusted domain check to init()

handleRequest() is not called from remote.php or public.php which made these files party available but all included apps in there produced errors.

As the expected behaviour is anyways that a trusted domain warning is shown I moved this to init()

Testplan:
- [x] Open a shared link from an untrusted domain without this patch => Works and gallery error is shown
- [x] Open a shared link from an untrusted domain with this patch => Trusted domain error is shown

Fixes https://github.com/owncloud/core/issues/10064

@MTRichards Please review.
@karlitschek I think we want to backport this to stable7.