Optimize offline map workflow

[nwagenbrenner]

The workflow for viewing offline maps is confusing and should be improved. Right now offline maps are turned on/off in the same place as the VIIRS, MODIS and other generic layers. An offline map is associated with a specific run, however, and so having the offline maps option here is confusing. For example, what happens if a user turns this layer on when they don't have any runs downloaded? Are all offline tiles (all tiles from all runs) displayed when the option is tuned on?