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)
I'm not entirely sure it should, it's just that there isn't an obvious way to detect whether the last run was minified or not, so I just specified that non-default behavior implies force. I can add a comment to that effect, or just fix the logic if I can figure out how to determine the previous value.