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 Once a comment is obsolete or uninteresting, you can hide it completely.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:

{F435621}

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

{F435626}

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

src/applications/differential/controller/DifferentialChangesetViewController.php

@@ -191,6 +191,7 @@ final class DifferentialChangesetViewController extends DifferentialController {
     if ($revision) {
       $query = id(new DifferentialInlineCommentQuery())
         ->setViewer($viewer)
+        ->needHidden(true)
         ->withRevisionPHIDs(array($revision->getPHID()));
       $inlines = $query->execute();
       $inlines = $query->adjustInlinesForChangesets(