Allow Challenge cancellation to be changed after a challenge closes

[ThomasKranitsas]

Current behavior

Currently, we do not allow changing the challenge status after the challenge has been completed.

Expected behavior

We should allow the status of a challenge going from Completed to Cancelled - Client Request

[ThomasKranitsas]

@marioskranitsas please createa PR against the develop branch

[marioskranitsas]

@ThomasKranitsas Done!