As a course creator, I want to be able to create a new relative dates course from an existing relative dates course so that I can reuse content in a new course

This issue is really about content re-use via the backup and restore systems.

Acceptance criteria:

• On restore, the course should keep the relative dates mode setting it had when backed up.
• [Assignment only] any assignment supporting relative dates should have its relative dates adjusted if the course start date is changed during restore.