Prepare TransactionCommentQuery for extension

Summary:
Ref T2009. Ref T1460. The way Diffusion and Differential load inlines is horrible garbage right now:

• Differential does an ad-hoc query to get the PHIDs, then does a real load to policy check.
• Diffusion completely fakes things. In practice this is not a policy violation, but it's dangerous.

Make TransactionCommentQuery extensible so we can subclass it and get the query building correctly in the right Query layer.

Specifically, the Diffusion and Differential subclasses of this Query will add appropriate withX() methods to let us express the query in SQL.

Test Plan: Loaded, previewed, edited, and submitted inlines in Differential and Diffusion

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009, T1460

Differential Revision: https://secure.phabricator.com/D12026