Allow only superusers to create/update/delete Location when Location Platform is Thoth

[brendan-oconnell]

We recently added a new Location Platform, THOTH, to the list of Location Platforms (see #649). This Location Platform is only to be used when the Location refers to files on a Publisher CDN hosted by Thoth, for Thoth Plus publishers.

For publishers participating in this program, the Thoth Location will always be canonical, and the URL will never need to be edited. Therefore, prevent accidental creation, update, or deletion of Thoth Locations by restricting this ability to superusers.