Move the bugzilla assignee update window to a modal
    
    - Add to the modal a button to reset the assignees to their default
      values
    - Add some explanations about the two input fields
    - Only show the EPEL-related information on the rpms namespace
    - Fix the layout of the different items in the side-bar while at it
    by:
      - Given them some top padding
      - Marking the titles as titles instead of paragraph
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>