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)
It took me a minute to work out why minify implies force, so I think a comment here would be useful.