Bug 1018707 - Empty CustomCSS

Moved the default link styles to be first in the declaration.

Moved the .ignore-external class out of customCSS and into wiki.styl. I will remove it from customCSS when this change is live.

There is a test page here: https://developer.allizom.org/en-US/docs/User:stephaniehobson:links (which is macro errors on staging but not on local or prod for some reason). The previous .ignore-external code was not actually working so we will see some changes around the site.