Fix creation of "subtask created" entries in parent task timeline
Summary:
`ManiphestTaskStatusTransaction::TRANSACTIONTYPE` is not used anymore when creating new tasks since rPd321cc810aab52be00d75c9dcfa8b9cabd34828e in 2017.
Among the transactions when creating a subtask is `ManiphestTaskParentTransaction::TRANSACTIONTYPE` which provides the data we're looking for, in order to reach the `if ($this->getMetadataValue('blocker.new'))` check in `ManiphestTaskUnblockTransaction::getTitle()`.
(`PhabricatorCoreCreateTransaction` does not provide `$unblock_xaction->getOldValue()` nor `$unblock_xaction->getOldValue()`.)
Closes T16275
Test Plan: * Go to a Maniphest task and select 'Edit Related Tasks > Create Subtask', create a subtask, look at the timeline of the parent task.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16314, T16275
Differential Revision: https://we.phorge.it/D26439