templates:

Abstracted the common template stuff from ghec and isc viewers into
a new template: `catalog_viewer`.
static/js/base_leaflet_app.js:

Added basic leaflet app template class. I want to reuse this in order
to reduce the duplication in our apps. >:(
templates/leaflet_base.html:

Added base leaflet app template.
templates/hazard_models.html:

Major cleanup to the hazard models template. Looks better.
static/js/hazard_models.js:

Removed a bunch of duplication and cleaned up the hazard app, based
on the new app template.