Log when an admin deletes/undeletes a collection through the admin

[diox]

Also avoid showing deleted field if the collection isn't already deleted - that's pointless and confusing, admins should use the regular delete action if they want to delete something.

Fixes https://github.com/mozilla/addons/issues/15394

Testing

• As an admin (needs Collections:Edit permission + access to the admin, or just *:*) look at a non-deleted collection detail page in the admin
• Deleted field should be absent
• Delete that collection through the button
• Collection should now be deleted - can't see it in frontend
• An ActivityLog with COLLECTION_DELETED action and the collection as argument should have been recorded in the database
• Go back to that collection admin detail page
• Slug field should not have changed (it would if the user would have deleted the collection themselves through the frontend)
• Deleted field should be present and ticked
• Untick Deleted field and hit save
• An ActivityLog with COLLECTION_UNDELETED action and the collection as argument should have been recorded in the database
• Collection should now be undeleted - can see it in frontend