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).

[mradamcox]

@rchampine and @parkerotto do you have thoughts on this idea, as it relates to the other discussion we had about reference layers? Would it be helpful to be able to see other years of maps in the georeferencing interface? Too confusing?

[rchampine]

Ooh yes I love the idea of being able to see other years. We are starting to do more editions for Richmond and this would be really helpful. I will echo what I said on the other thread about reference layers to reiterate that it's useful to separate the different types of reference layers into unique stacks or control groups on order to keep the hotkeys efficient (i.e., hotkeys should toggle opacity % 0/50/100 on one layer type and not have multiple layer types).

[parkerotto]

I think seeing other years would be amazing! Especially for corrected Sanborns (like how 1950s ones are corrected versions of 1926ish ones). I don't think that it should be toggled on by default, though. Perhaps a new hotkey would be appropriate (like 'h' for history, or something).