Make aphront-side-nav more clear re what's a link

Summary:
Create a visual hierarchy with the <span>s and <a>s in the aphront-side-nav
so people don't try to click on a span thinking it's a link. This is to
help specifically with the case of the "All Revisions" header on the
differential revision list page - I've had a few people ask about that
broken link.

Test Plan:
Loaded the differential revision list view and the maniphest task list
view to check that their left-hand navs look ok (did this in ff8 on ubuntu
11.10).

Reviewers: epriestley, jungejason, btrahan

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: https://secure.phabricator.com/D1303