Add CSS classes for diff/hist links at Enhanced RC

Add CSS classes for diff/hist links at Enhanced RC

CSS classes mw-changeslist-diff and mw-changeslist-history should be
added to diff and history links in recent changes. There is already a
patch for the old recent changes page, this one adds the same classes
onto the enhanced recent changes page (class EnhancedChangesList). As
enhanced RC has a lot more links, this patch introduces two new css
classes for some links, one named  #mw-changeslist-diff-cur which is for
the diff that links to a current version within nested changes and the
other named #mw-changeslist-groupdiff which is for the 'summary diff'
that appears for a group of nested changes.
Follow-up: I2d5ef8c180ae4ff6e7f5d0ab443dc7084f8c4c77

Bug: T157178
Change-Id: Ib51639a92b5925f2bad0aebd4f7068b178f65017