if a review for the maintbot user is added for a second time the old
function would only see the 'accepted' state of the first review and
the bot would not change the review state.
factory-source checks if submissions contain sources already accepted in
Factory. maintbot makes sure maintenance submissions are authored by the
Factory package maintainer.
Both are based on a new generic ReviewBot.py class that can serve as
common framework for review bots.
