Allow reassignment of withdrawn review assignment

[rjsparks]

From a review team secretary:

I assigned a review to someone
That individual did not response in a timely manner so I withdrew the assignment
I assigned to another reviewer (new reviewer)
The original reviewer then indicated they could do the review
I withdrew the review from the new reviewer
I attempted to assign to the original review, but cannot

I worked around this by making an adjustment to the withdrawn assignment using the admin.
The team secretary should be able to make the assignment again (just move the state from withdrawn to assigned).

[rjsparks]

Also note that when the reassignment is made, the ReviewRequest itself should transition to the assigned state.