Refactored the way constants are passed to client

The leaflet base map url is now a constant in the local_settings.py.

Through some refactoring in templates and leaflet app code, this url
is now available as a global constant to all leaflet-based apps
(at least ones that use the leaflet_base.html template).
Refactored URL constant injection
explore:

Fixed some spelling errors.
templates/calculate:

Removed a redundant copyright/license header.
Added jquery.form to leaflet app includes
static/js/calculate.js:

Implemented basic run-calc functionality in the calculate UI.
Merge pull request #100 from larsbutler/basic-calculate-functionality

Basic calculate functionality

[r=matley] [f=1234765]