Allow inline comment storage objects to generate their own runtime objects

Summary:
Ref T13513. Currently, inline storage objects ("TransactionComment") can't directly generate a runtime object ("InlineComment").

Allow this transformation to be performed in a genric way so clunky code which does it per-object-type can be removed, lifted, or simplified.

Simplify an especially gross callsite in preview code.

Test Plan: Previewed inline comments.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21226