When generating Calendar event stubs, inherit import properties

Summary:
Ref T10747. Previously, importing a recurring event failed to mark the instnaces of the event as imported.

Now, we copy the source/UID/importer over.

Test Plan: Imported a recurring event, viewed event series, saw all of them marked imported.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16728