[Task] Make core app independent of georeference app

[mradamcox]

Describe the task
The core app should have migrations that do not depend on any other app. Currently, the LayerSet model is in the georeference app, where it made sense to put it at the time it was created. However, it should be moved within the core app. Similarly, core.Region has a foreign key to georeference.GCPGroup but that relationship can be reversed.

Completion criteria

• New LayerSet model in the core app created
• Copy all existing objects to that model, update code
• Remove LayerSet from georeference app
• Create ForeignKey on georeference.GCPGroup to relate to core.Region and populate with all existing values.
• Update code, then remove existing core.Region.gcp_group relationship

Additional context
This reorganization makes more sense thematically, as LayerSet is just a group of Layers and needs to have a first-class (in-app) connection back to the Map model which is in the core app. More importantly, this should make proper squashing of the existing migrations #205 (which will elide models that have been created and later deleted) possible. This ticket overall will finally, properly, complete the migration to the new data model.