Replace hardcoded "modular" support with generic "dist" support
    
    We only ever did two Fedora-Modular composes, in the end, so all
    the effort I put into supporting a parallel set of events for
    modular composes seemed wasted. BUT! We now want a parallel event
    stream for IoT composes: https://pagure.io/fedora-qa/issue/623
    So, we can achieve that by making all this code to handle modular
    composes more generic. Instead of hardcoding the concept of a
    "modular event" for modular composes, we just kinda allow for
    the existence of event streams for any given "dist", where a
    "dist" is a Pungi compose shortname like "Fedora", "Fedora-Modular"
    or "Fedora-IoT". "Fedora" as the dist should give all the same
    page names and contents as you previously got with modular=False.
    Anything else as the dist will give you page names, contents and
    so on based on that dist name; often it is used unmodified, and
    sometimes we use a form with the "Fedora-" prefix stripped. Note
    that "Fedora-Modular" as the dist will not give you exactly the
    same page names and contents as modular=True previously did,
    because implementing that involves doing a lot of grandfathering
    in both this code and the wiki templates themselves. Instead, we
    are going to rewrite history: I will rename and edit all of the
    (few) existing wiki pages and templates for modular validation
    events (we only ever did two, only nightly and one candidate,
    for Fedora 27), so they match what this new code generates.
    Probably no-one but me cares about those pages anyway...
    
    We will need to tweak some of the actual wiki templates a bit
    for this, too, and adjust relval and relvalconsumer to the new
    way of doing things.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>