Moves Openlayers layer switcher styling overrides into public payload, tweaks.
Only display the layer switcher widget when the map is using a spatial layer, since it's useless in exhibits using static-image layers (of which there can never be more than one).
Adds zoom_levels field to exhibit model, implements on exhibit add/edit form.
Validate integer on zoom_levels.
Standardizes SQL query formatting in migrations.
Blocks in 2.1.2 migration.
Bumps version.
Fixes require path for 2.1.2 migration.
Adds default value for 'zoom_levels' in migration.
Committing compiled assets.