Give "create" transactions a readable type in "transaction.search"

Summary:
Ref T13151. See PHI725. By default, "transaction.search" doesn't provide details about transactions because many have bad/weird/policy-violating internal types or fields.

The "create" transaction is simple and straightforward, so label it to allow callers to distinguish it.

Test Plan:

• Created a new task.
• Called transaction.search on it.
• Saw the labelled "create" transaction.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: swisspol

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19505