find ComposePage from wiki page, give ComposePage a sort key
    
    ComposePage.from_wikipage() is a class method that you can feed
    an mwclient page object, and it'll try to spit out the matching
    ComposePage object.
    
    To make this practical, I had to make it so ComposePages don't
    have a 'parent' event object that they take various attributes
    from any more - you just have to pass a release, milestone and
    compose when instantiating one.
    
    sortname() is a ComposePage instance property that can be used
    for sorting sets of ComposePage objects (it just tweaks the
    milestone and compose atrributes into a form that will produce
    the correct order - Alpha < Beta < Preview < Final, Pre- before
    (not Pre-), TC < RC - when sorted alphabetically).