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.