Put the keyboard focus reticle back on top of diffs

Summary:
Fixes T10954. This got hidden underneath things at some point.

Use pointer-events: none to make the mouse ignore the element so that hover/select/edit/click still work "through" the element.

Design could probably be improved here, maybe I'll make it more-visible when you press n and then have it fade quickly so it kind of gets out of your way once you find the block you want to read.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10954

Differential Revision: https://secure.phabricator.com/D17784