R1:5cb0de1efc31

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…