Make sure the url pattern descriptor exists for every main tab

Since NXP-7209, we can select a pattern descriptor on POST, and the same pattern descriptor should be used to encode main tabs links.
This is done using a hack: the system tries to get the pattern descriptor with the same id than the action behind the main tab.

There is currently no check to verify that a pattern with that name exists => new API is needed on URLPolicyService for it.