fabfile:

Added `artifact_groups` url to client constants.
static/css/oqplatform_leaflet.css:

Make leaflet layer control panels scroll if there are a lot of layers in the list.

Otherwise, the layer list runs out of bounds.
templates/calculate.html:

Minor clarification to a comment.
static/css/oqplatform_leaflet.css, templates/leaflet_base.html:

Some minor tweaks to allow easy display of "Loading..." dialogs
with an ajax spinner.
calculate.js:

Completed calculation loading workflow and implement a super
basic prototype of visualization. This is REALLY REALLY rough
and needs a lot more work.
static/js/calculate.js:

Added a warning about some nasty code, related to a closure
and some subtle scoping.
templates/leaflet_base.html:

Removed some commented-out html.
Merge pull request #101 from larsbutler/view-calculations

`View calculations` prototype
[r=micheles] [f=1236843]