Cache only 200 results in /assets/*.
Cache all assets.
Move all assets to CDN.

Note that while rebasing this commit, I (@seanlinsley) added
a good deal of extra changes since we were still using
the static "/assets/foo" URL in a lot of other places.
Fold chosen.css and reset.css into gittip.scss.
remove unused assets
version all assets

This doesn't version the old widgets because people may still be using
them. (ref: #1767) Except... the first widget refers to an old version
of jQuery that we haven't provided in a long time :-/

This commit also fixes reset.css and chosen.css to correctly load.
remove now-unused `asset_url`

and start using that name for `asset_version_url`
use `asset_url` for elsewhere icons
turn `get_avatar_url` into a Jinja macro

that way it has access to the website object by default
Merge pull request #1969 from gittip/fix-cache

Cleanup assets serving