Use the require.js optimizer

Use django-require to bundle Widgy's js during collectstatic.

`widgy-main.js` is a stub that gets turned into the widgy js bundle
during collectstatic. This allows us to always include it during
development. Since it doesn't do anything, require.js will work as
normal (lazily loading everything), until the bundle gets built.
Fix css paths to make it work with CachedStaticFilesStorage
cachebust require.js in development