Fix left/right detection of inline comments in unified view

Summary:
Ref T2009. Currently, the code figures out if a comment is on the left or right by looking at the <th /> preceeding the enclosing <td />.

This gets the right result in 2-up, but in 1-up rows are always <th />, <th />, <td />, so it always detects every inline as being in the new file.

Because "old" and "new" cells aren't inherently distingushable in the 1up view, we can't use a DOM test for this at all. Instead, just track this state explicitly.

Test Plan:

• Made left/right comments in 1up view and 2up view.
• Viewed them in 1up and 2up views.
• Hovered in 1up and 2up views.
• Diff-of-diff'd and reviewed old/new comments, then made some more.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12011