Reduce code duplication for inline "Undo"

Summary:
Ref T2009. This is another almost-identical copy of the row scaffolding, which has the same 1up/2up bugs as the 8 other copies of this code.

Turn the "undo" element into an InlineCommentView so we can scaffold it.

Then, scaffold it with the same code as everything else.

Test Plan: Hit "Undo", swapped from 1up to 2up, hit "undo" again, swapped back, tried left/right, everything rendered with proper scaffolding.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12019