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

src/applications/transactions/conduit/TransactionSearchConduitAPIMethod.php

@@ -253,6 +253,10 @@ EOREMARKUP
                 break;
             }
             break;
+          case PhabricatorTransactions::TYPE_SUBSCRIBERS:
+            $type = 'subscribers';
+            $fields = $this->newEdgeTransactionFields($xaction);
+            break;
         }
       }