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