Implement multi-level internal reference layers in georeferencing interface

[mradamcox]

When georeferencing a sheet from a volume...

• The key map from that volume should be available as a reference layer in the georeferencing interface. This is very helpful for the user if they are trying to locate a sheet within a mid-size to large city.
• Further, a second "level" of reference layer(s) should be made available, all of the already georeferenced sheets from the given volume.
• Further still, other maps from the same place (i.e. editions from other years) should be made available as reference layers as well.

This will require a sleeker layer management panel than the georeferencing interface has had in the past, so figuring out a UI design is really the main part of this ticket (passing the layer urls around shouldn't be especially difficult, using the volume.locale property to find related layers).

[mradamcox]

This is somewhat similar to #89, in that the design here should account for that ticket, but this ticket need only concern internal layers, i.e. other georeferenced content in this platform (which will be much easier to work with).