Closes #169: Disable map view and show message if no nearby locations

As requested by @jen-tolentino, the app will now hide the map view and
disable the map switcher if the selected filter type has no locations.

If in the map view and a switch to a filter with no locations occurs,
the map view will hide and disable the button.
Merge pull request #181 from CloudNiner/feature/hide-map-if-no-locations

Closes #169: Disable map view and show message if no nearby locations