First pass at modalizing source selection.
Remove debugging.
Go green.
Get active source state working.
Load tilesets into history from mapbox API.
Merge branch 'master' into source-modal
Merge branch 'master' into source-modal
The big refactor:

- Only show local sources in history megamenu,
- Split change source modal in to local/remote tabs,
- Remove add mapbox modal, incorporate into change source modal.
Get mapbox source thumbs working.
Stronger active state on items.
Unit tests for userTileset middleware.
Less rigid check for userTilesets history entries.
Merge pull request #349 from mapbox/source-modal

Source modal