Write one DifferentialComment per CommentEditor action

Summary:
See D8200. Ref T2222. Instead of writing one comment which can have a ton of different effects, write a series of one-effect comments. These will be easier to convert into ApplicationTransactions.

This has a minor user-facing effect of making these multiple-action comments render separately:

{F111919}

Once the migration completes, they should automatically merge together nicely again.

Test Plan: Made a bunch of comments and took a bunch of actions, all of which worked normally except that they rendered as several things instead of just one.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, FacebookPOC

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8201