Support 'Fedora-Modular' validation events
    
    For the Fedora 27 cycle, we decided to split off development
    of the 'Modular Server' edition from the rest of Fedora 27. As
    a consequence of this we have a separate 'Fedora-Modular'
    compose stream, with nightly and candidate composes for the
    'Modular Server' edition, on a different release schedule to
    the main composes. To support validation of the Modular Server
    release, we need to have a parallel stream of validation events
    which doesn't interfere with the main stream of Fedora 27
    validation events.
    
    This giant commit achieves this. Various relevant `Event` and
    `Page` subclasses now have an optional `modular` arg; if this
    is truth-y, the event or page will be for a modular compose.
    Page and category names have 'Fedora Modular' in them instead
    of just 'Fedora'. The `Release` class also has an optional
    `modular` arg; if this is truth-y, `milestone_pages` will find
    pages for Modular events for the given release, not 'regular'
    events. On the wiki side I've added modular equivalents for
    `CurrentFedoraCompose` (`CurrentFedoraModularCompose`) and
    the `Current` redirect pages; python-wikitcms will update these
    when creating modular composes, rather than the non-modular
    pages. `get_validation_page` and `get_validation_event` also
    get a `modular` argument that can be set truth-y to search for
    a Modular page / event; they will also automatically do the
    right thing when passed a Modular compose ID. Finally, the
    page generator bits in `listing` are enhanced to recognize
    Modular page names and return appropriate `Page` subclass
    instances.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>