Modern versions of Uglify don't mangle (alter code to shorten variable

names and logic) by default, which can mean losing out on some nice

reductions in file sizes. For our stuff, we want to default to mangling.

While this make a small difference for Djblets (about a 90KB savings),

it can make a much larger difference for consumers.
The arguments now include --compress --mangle, just to be thorough.

This is set automatically in build_pipeline_settings(), and can be

turned off if required.

Successfully built static media for Djblets and Review Board and tested

it across many areas of the sites.