Add local navigation to Differential

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