Create new base layer for each map

This should fix the browser back button for #50. The problem is that
for each partial page load a new L.Map is created, but only one
baselayer is shared across all L.Map instances. This also obviates
the need to dump and reload all layers. An entirely contained L.Map
object exists for each page, without shared context.

As mentioned in #72, I think there are some larger underlying issues
with how we're managing the partial page loads in Turbolinks. The
issue this commit addresses is related to #72. In the long run, I
think we are going to need to rethink our approach, but this commit
should close out this issue.