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