Added production builds to Grunt watch.

To avoid my continually forgetting to general .min.js and .min.css files
before pushing and deploying. It's annoying to generate these files on
every save, but it's so quick that it doesn't really matter anyway.
Moved attachment page styling to a separate file.
Simplify CSS build by using a separate cssmin task.

Quicker than running the LESS compile twice for the "prod" cleancss step
and more straightforward anyway.