Automatically reassign a case if it hasn't been closed in the required timeframe

[jpaidoussi]

As per OPENHELPD-62 a case needs to be automatically reassigned to its previous assignee if it hasn't been closed after the configured amount of time has passed since it was reassigned to the current partner / user.

Potential steps required:

• Add Django setting for case response required time
• Add lookup for cases that are potentially past their required response time
• Add periodic task to lookup expired cases
• Update CaseAction to allow null created_by field
• Update CaseAction methods to fully support blank user (created_by) parameters
• Add task to reassign case if it is expired and add note
• Add tests
• Look into UI changes required to support null created_by field for CaseAction.