Make less/sourcemaps a bit friendlier to git

Hopefully reduce some pain caused by #6149.

- remove sourcemaps from repo
- don't track minified css
- build sourcemaps in post-checkout/merge hooks
- try to build minified css and sourcemaps on install (warn on failure)
- build minified css and sourcemaps on release (die on failure)
- don't run lessc if less files haven't changed (force to override)
`open(blah).read().count('\n') > 100` ;-)

I think a comment is fine. We'll probably be doing minify rarely enough that it will always need to run if minify is True.