Admin enhancements

* Prevent admins from deleting a corpus using the admin,
  force them to use the deletion API instead
* Add type filter on metadata
* Remove revision from metadata list (100 less queries!)
* Add corpus filter on entity roles
* Show element type display name instead of ID in the elements admin

This will probably not prevent 500, 502 or 503 errors in prod, as that
is just caused by a lack of hardware resources to handle the amount of
data we have.
We could also have a policy of removing from the admin all the actions
that can be done using the API? That would "hide" a lot of issues.