Reverse light and dark distribution map shades.

Now county-level records get the darker shade. (#577)
Increase shade contrast for distribution maps. (#577)
Add a link to show map legend definitions. (#577)
Replace text below the map, linking to New Sighting. (#577)
Fix a failing test after change to map colors. (#577)
Arrange the county-level map legends into categories. (#577)
Edit copy under county-level map about posting a sighting. (#577)
Update map tests so all pass again. (#577)
Adjust spacing around maps. (#577)
Rebuild production JS file.