Summary:
Adds a flexible navigation menu to diffs that shows you your current position in the diff.
Anticipating some "this is the best thing ever" and some "this is the wosrt thing ever" on this, but let's see how much pushback we get? It seems pretty good to me.
Test Plan: Will attach screenshots.
Reviewers: vrana, btrahan
Reviewed By: vrana
CC: aran
Maniphest Tasks: T1633, T1591
Differential Revision: https://secure.phabricator.com/D3355
