Store metadata with Differential and Maniphest comments, and store added

reviewers as metadata

Summary:
The "Add reviewer" implementation is super lazy right now since I didn't want to
do a schema change. Man up and add a column. I also plan to store "via"
information here (e.g., via email or via mobile).

NOTE: This schema change may take a while since the comment table is pretty big
in Facebook's install.

This needs a little CSS work but I think it's reasonable for now.

Test Plan:
Made comments on revisions and tasks. Added reviewers to a revision, got linked
names instead of a blob of text.

Reviewed By: aran
Reviewers: tomo, jungejason, tuomaspelkonen, aran
Commenters: tomo
CC: aran, tomo, epriestley
Differential Revision: 394

src/applications/differential/controller/revisionview/DifferentialRevisionViewController.php

@@ -78,6 +78,19 @@ class DifferentialRevisionViewController extends DifferentialController {
         $user->getPHID(),
       ),
       mpull($comments, 'getAuthorPHID'));
+
+    foreach ($comments as $comment) {
+      $metadata = $comment->getMetadata();
+      $added_reviewers = idx(
+        $metadata,
+        DifferentialComment::METADATA_ADDED_REVIEWERS);
+      if ($added_reviewers) {
+        foreach ($added_reviewers as $phid) {
+          $object_phids[] = $phid;
+        }
+      }
+    }
+
     foreach ($revision->getAttached() as $type => $phids) {
       foreach ($phids as $phid => $info) {
         $object_phids[] = $phid;