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 always call `setup.py css` to rebuild the css, which should not minify by default. It's just in the release/install steps, which imply minified css.