Redirect only to absolute URL

We do not want to redirect to other domains using the "?redirect_url=" feature. Please notice, that the ownCloud project does not consider open redirectors as security issue.

To test:

- [ ] http://localhost/core/index.php?redirect_url=%2Fcore%2Findex.php%2Fsettings%2Fadmin redirects to admin page
- [ ] http://localhost/core/index.php?redirect_url=https://www.owncloud.com redirects to invalid page on same domain

Fixes https://github.com/owncloud/security-tracker/issues/73