Make DifferentialChangesetParser explicitly map display to storage for comments

Summary:
This is a followup to D228. Basically, we use "changeset" (and, implicitly,
changesetID) for way too much stuff now.

One thing it can't possibly capture is the complete, arbitrary mapping between
the left and right sides of the displayed diff and the places we want to store
the left and right side comments. This causes a bunch of bugs; basically adding
inline comments is completely broken in diff-of-diff views prior to this patch.
Make this mapping explicit.

Note that the renderer already passes this mapping to
DifferentialChangesetParser which is why there are no changes outside this file,
I just didn't finish the implementation during the port.

This has the nice side-effect of fixing T132 and several other bugs.

Test Plan:
Made new-file and old-file comments on a normal diff; reloaded page, verified
comments didn't do anything crazy.

Expanded text on a normal diff, made new-file and old-file comments; reloaded
page, verified comments.

Repeated these steps for a previous diff in the same revision; verified
comments.

Loaded diff-of-diffs and verified expected comments appeared. Made new left and
right hand side comments, which almost work, see below.

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran, ola
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 237