Restore "Create" transactions

Summary:
Ref T10004. This restores "alice created this task." transactions, but in a generic way so we don't have to special case one of the other edits with an old null value.

In most cases, creating an object now shows only an "alice created this thing." transaction, unless nonempty defaults (usually, policy or spaces) were adjusted.

Test Plan: Created pastes, tasks, blogs, packages, and forms. Saw a single "alice created this thing." transaction.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10004

Differential Revision: https://secure.phabricator.com/D14820