Add tab for candidate compose / stable push request text
    
    Summary:
    This adds a 'Requests' tab which provides proposed text for a
    candidate compose request and a stable push request for the
    current milestone, in wikitext format as these requests are
    currently sent to Trac. We'll probably have to change it to
    Markdown for Pagure soon enough, but that would be easy.
    
    The candidate compose request lists all updates that:
    
    * Are not stable, obsolete or deleted
    * Fix an accepted blocker or FE bug for the relevant milestone
    
    along with the related bugs. The stable push request adds one
    more condition to those two:
    
    * Is queued for stable
    
    In both cases, each update is listed along with the bugs it
    fixes, in the same basic format I've been using for these
    requests (doing it by hand) for years. The text is intended as
    a starting point for a human to review, as sometimes it'll need
    tweaking for unusual circumstances.
    
    I included tests but I can't seem to run the tests here (pytest
    blows up with some weird error when I try), so I haven't tested
    the tests...I hope they work. :)
    
    Test Plan:
    Run the app, go the the Requests tab for a milestone
    (Fedora 25 Final is the best to try at present), check the text
    looks appropriate.
    
    Reviewers: tflink, mkrizek
    
    Reviewed By: mkrizek
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1052