R1:1352be827e12

Begin separating inline comment scaffolding from other renderers

Summary:
Ref T2009. Inline comments have "scaffolding", which is basically some empty table cells/rows around them to get the layout correct.

The scaffolding depends on the renderer, since the cells are different for side-by-side vs unified diffs.

This is currently duplicated all over the place:

- Edit view has 1up/2up.
- Detail view has 1up/2up.
- 1up renderer has 1up.
- 2up renderer has four separate copies of the 2up logic.