Fill <a href> in PhabricatorMenuItem

Summary: The <a href> attribute is useful because user knows where the link goes
before opening it plus he can copy it to the clipboard plus he can add it to the
bookmarks.

Test Plan:
Display revision.
View Options.
Click.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D1436