Expose subscribers transaction data via Conduit "transaction.search"

Summary:
Depends on D20507. See PHI1232. Previously, see T13255 and D20209.

Since nothing seems to have exploded after "projects" was exposed, give "subscribers" the same treatment.

Test Plan: Added, removed, and modified subscribers. Queried transactions with "transaction.search", saw sensible "type" and data.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20508