Map on /brigades/* page

First pass on adding a map to the /brigades/* page - places a map below the list of locations, which automatically centers / expands to include all locations listed on the page.

Markers get an icon based on whether they are In Progress or Deployed --- currently using a couple of generic icons.

Sample map: http://brigade-map-test.herokuapp.com/brigades/3

Maybe it would look better with clusters?

Fixes #213