Prefix even chained classes with "mw-ui-"

Kaldari points out that without this, user or wiki CSS names like
"primary" will affect the Agora styles.  Thus class="mw-ui-button big
block primary" becomes class="mw-ui-button mw-ui-big mw-ui-block
mw-ui-primary".  It's more verbose, but cleanly separated.

Change-Id: I35f6e58311368deaaa4595ab3459cd22c5c57e18