Abstract access to DifferentialInlineComment behind a Query

Summary:
Ref T2222. See D6260.

Push all this junk behind a Query so I can move the storage out from underneath it.

Test Plan: Viewed home page, list view, revision. Made draft, looked at preview, submitted draft, viewed inline, replied to inline.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6262