Viewer performance

[mradamcox]

Two changes to how content is furnished to the viewer component, finally improving the initial page load in a very meaningful way.

1. Creates a new schema for LayerSets that provides a single place to get a definition of the layer mosaic, whether it has a single geotiff or still has many different layers, each with their own mask.
   • Better utilize tilejson on each layer, and the new Layer.mask field (no longer uses LayerSet.multimask)
2. Use a much lighter-weight schema for the Maps. This could probably be pared down further but no need to do that now.

Quick, non-exhaustive benchmarks:

locale	timing before	timing after
New Orleans, La.	16.83s	2.16s
San Francisco, Ca.	20.03	2.53s