Extract LESS into one file per-widget

Summary:
Instead of having one big perseus.less, this commit extracts the styling
for each widget into its own file. This should make it a bit easier to
find CSS for a specific widget.

It also modifies operations.sh to precompile the LESS into CSS before
copying it over so we still only copy over one file. There's an
associated commit with webapp that makes it use perseus.css instead of
perseus.less.

LESS is also added as a dependency in package.json, so that it is
guaranteed to be installed when running `make all`

Test Plan:
* Open up this hodgepodge exercise with everything in it:
  https://gist.github.com/justinj/51288aaba60966731e5a
* Ensure all the widgets still work

Reviewers: alex, jack

Reviewed By: jack

Subscribers: joel, eater, brianmerlob, alpert

Differential Revision: http://phabricator.khanacademy.org/D8100