Couple of fixes around milestones and priorities
    
    This commit does a couple of fixes:
    a) forward issue.milestone to the backend when calling edit_issue() so
    that we do not remove the milestone to add it back shortly after
    (triggering two fedmsg notifications one of which is invalid)
    
    b) Allow the priority to be None, so all priority information of a ticket
    can be removed and fix making the current priority the selected one in
    the form displayed in the UI