Compress text/javascript files, too

This was preventing large files like `bundled.js` from being compressed over the wire. They were being sent with `Content-type: text/javascript` instead of `Content-type: application/javascript`.

As for what they _should_ be sent with, it seems [murky](http://stackoverflow.com/a/4101763/388951).

The `COMPRESS_DEBUG` param enables gzip for the local Flask dev server. Seems very 12 factory.
What does it do? Seems we do have special functionality vs production when DEBUG is true for other things.