Add PHIDs to DifferentialComments

Summary:
Ref T2222. This adds PHIDs to all Differential comments so I can migrate the inlinecommment table to transaction_comment in the next diff.

@wez, this will issue a few million queries for Facebook (roughly, one for each Differential comment ever made). It's safe to skip the `.php` half of the patch, bring Phabricator up normally, and then apply this patch with Phabricator running if that eases the migration, although the next few diffs will probably be downtime-required migrations so maybe it's easier to just schedule some downtime.

Test Plan: Ran migration locally. Verified existing comments and new comments received PHIDs.

Reviewers: btrahan

Reviewed By: btrahan

CC: wez, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6266