Split "/about/" out; deduplicate template context

Fixes issue #211

Rather than using javascript hacks to hide/show the #about div, separate
about and index into separate templates.  To deduplicate, also separate
the sponsorship message and analytics into template stubs so they can be
included in each of the views rather than duplicated.

(An alternative approach that seems heavier than necessary here would be
to invoke a Backbone.Router and unify index/about/how-to-hangout).

On the server, make use of "express.locals" to provide default context
(e.g. {_:_, user:undefined}) to all templates, simplifying "render"
calls.  Remove unused template vars (e.g. "loadApp").