Remove regions

Removes Region and RegionElement and uses DeprecatedMixin to cause 410 Gone on all region endpoints.

Migration steps and timing on a preproduction dump:

• Add Region.element foreign key: Negligible
• Fill foreign key: 3 minutes
• Remove RegionElement: Negligible
• Migrate regions:
  • Create element types: 4 seconds
  • Create elements: 90 seconds
  • Create classifications: 2min15
  • Create paths for regions on elements without paths: 11 seconds
  • Create paths for regions on elements with paths: 1min10
  • Commit transaction (disk writes): 11 minutes
• Remove Region: Negligible