R1:62bc3373e5a6

Fix error with inline comments on images

Summary: This data structure is a `dict<int, list<Comment>>` now, where the `int` is the line number.

Test Plan:
- Created a diff changing an image.
- Added inline comments on the left and right sides of the diff.
- Saw some exceptions and general sadness.
- Applied patch.
- Reloaded page.
- Everything worked great.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4264