Allow inline comments to be individually hidden

Summary:
Ref T7447. Implements per-viewer comment hiding. Once a comment is obsolete or uninteresting, you can hide it completely.

This is sticky per-user.

My hope is that this will strike a better balance between concerns than some of the other approaches (conservative porting, summarization, hide-all).

Specifically, this adds a new action here:

Clicking it completely collapses the comment into a small icon on the previous line, and saves the comment state as hidden for you:

You can click the icon to reveal all hidden comments below the line.

Test Plan:

• Hid comments.
• Showed comments.
• Created, edited, deleted and submitted comments.
• Used Diffusion comments (hiding is not implemented there yet, but I'd plan to bring it there eventually if it works out in Differential).

Reviewers: btrahan, chad

Reviewed By: btrahan

Subscribers: jparise, yelirekim, epriestley

Maniphest Tasks: T7447

Differential Revision: https://secure.phabricator.com/D13009