Allow events from a particular import source to be bulk-deleted

Summary:
Ref T10747. If you accidentally import the wrong thing, you can clean up the big mess you made.

These imported events are read-only so it's OK to destroy them completely (vs disable/hide/archive).

Test Plan: Destroyed some imported events.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16720