Add is_valid property to assignments?

Pulled out of #35

Use case:

> Cathy and Anna are both Deputy Finance Directors. Contracts can be approved by either of them during the "Awaiting Finance Approval" state.

@mjumbewu said:

> I'm considering adding an is_valid field to Assignment instances. So, in the case where Cathy and Anna are both Deputy Finance Directors, when one acts on their assignment, the other's assignment would get is_valid set to False -- perhaps explicitly in the handler, perhaps automatically as a consequence of the case transitioning out of the assigned state with no action taken on the assignment. Probably the latter.
>
> Does that seem reasonable? If the case transitions to another state, then all incomplete assignments are simply marked as invalid?

@timwis replied:

> maybe I'm misunderstanding, but I'm not sure that's necessary. If the /cases/awaiting/ endpoint queries cases that (a) are assigned to you, (b) you haven't acted on yet, and (c) the case is currently in the state you're assigned to -- it should handle both scenarios described above, no?