Ensure only valid claims can be completed

In investigating #87 I noticed that there was no validation that a claim existed and was valid when it was completed.

I have asked some questions in #90 to clarify what the intended behavior is, but I believe that it shouldn't be possible to complete a claim that is not currently valid.
I noticed there is no validation anywhere that requires a completion be related to a currently valid claim. Is this a check worth doing somewhere?

I had assumed that a client could only complete a claim that was active (otherwise someone else could be currently working on that task)