shared.css: Limit .plainlinks styling to .external links

Overriding the background of all links on the page can have unexpected
results, for example causing the "Enlarge" icons under thumbnails to
disappear when the entire page is wrapped in <div class="plainlinks">.
https://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Signpost/2014-08-27/In_the_media

Change-Id: I6fb15c5de39419993d37d9f5dd31ecb7ae6503fa
Merge "shared.css: Limit .plainlinks styling to .external links"