Allow g.cache_chains to be extended after creation.

Cache chains added in plugins need to be reset, too!
Add extra_stylesheets knob to Reddit.

This provides an easy way for plugins to add extra separate stylesheets
to the site.
Add Backbone 0.9.10 to the main JS.
Add util for locating static files. Move to static.py.
Search static directories in a consistent order.
Whitespace.
Clean up file reading in js.py.
Add simple compiled in template support.
Upgrade underscore.js to version 1.4.4.
Add JSPreload system for including JSON data at the bottoms of pages.
Hover bubble UI overhaul.

* Backbone
* Show event
* Display groups (for fast show/hide switching)
* Right alignment mode
* Slide animations
Hover Bubble: add .anchor-right-fixed positioning mode.

Like anchor-right but using fixed positioning and clamped to the window
dimensions.
Prevent space compression of inline script tags.
Add hook for finalizing js_preload.

Useful if things generated in multiple places need to be coalesced into
a single preload URL.
Upgrade to Backbone 1.0.0.
Fire jQuery event when creating new thing elements.
Fire jQuery event when expanding expandos.
Fire jQuery new thing events for thing insertions as well.