Remove possibility to delete movies

Since there's no manual adding of movies this is not needed. Also, this
would have had to been access restricted, and we don't have a way to tell
admins apart from regular users yet.

Might be added again later.

Tests and docs also removed.
Remove unused marvin.validators
Remove stale reference to POST endpoint for creating movies
Add test for updating invalid entries
Restrict entry creation to the owner of the stream