Make the list of banned username configurable

[mscherer]

At https://github.com/fedora-infra/fedbadges/blob/develop/fedbadges/rules.py#L100 , we have a list of username that shouldn't get badges. It should be a configuration option rather than be hardcoded in the code.

My use case is that mbs/mbs.fedoraproject.org is sending messages on the bus, and it seems that this is overloading fedbadges who try to do a query.all() on all the build made by that user, which is "a lot".

Right now, I am testing a patch that add it to the list of banned user, but ideally, it shouldn't requires to change the code, just a configuration setting.