Syncing mobile header fonts with new Vector skin stack

Although we could theoretically just reuse the LESS variable in
core MediaWiki, I think it's likely we may want to diverge from
core again at some point in the future so it won't hurt to keep
using our own variable for it.

Change-Id: I59cc97c7e6c31d7108f6c82aeaa30c373b42f2c0
Merge "Syncing mobile header fonts with new Vector skin stack"