Do not expire requests which are assigned to somebody

In Launchpad, we use the support tracker to track administrive kind of requests. It is possible for requests to take more than two weeks to be completed (because accomplishing the task might be blocked on something else). For these use cases, there should be a way to prevent expiration of these requests.

A good way to signal that a request doesn't need expiring is with assignment. An support request that is assigned to somebody shouldn't be expired whatever its state.