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.

These all have subtle differences, which are mostly bugs. Start making the scaffolding more composable so we can get rid of that mess.

Test Plan: Added, edited, and removed inline comments on unified and side-by-side diffs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11997