discussions: ignore closed tickets
    
    I believed we ignored closed tickets already, but we probably forgot about it.
    It is implemented now.
    
    At the same time, when we close old tickets, make sure we close it first and
    post a comment later, otherwise we still trigger the webhook and we often don't
    want to do that.
    
    As a bonus, cover the whole webhook API with tests.
    
    Merges: https://pagure.io/fedora-qa/blockerbugs/pull-request/167