handle 0-day and stable blockers
    
    Summary:
    As per discussion on test@, we'll be using a couple of new magic
    whiteboard strings to identify two sets of 'special' blockers:
    Accepted0Day and AcceptedPreviousRelease. The first are bugs that
    must be fixed in the 0-day update set for the new release. The
    second are bugs that must be fixed in updates for the previous
    stable release(s) before the new release goes out. This commit
    allows blockerbugs to handle them, displaying them as two new
    groups on the Bug List and IRC Format pages.
    
    Test Plan:
    Check that the new blocker types are correctly detected
    and listed and the app works as before in all other respects.
    
    Reviewers: tflink, mkrizek
    
    Reviewed By: tflink, mkrizek
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D689