Remove the check that the agenda is free in the future when adding or editing a meeting

This check was making sure that the agenda (which until now was constructed more as a
meeting room) was free in the future for the dates and times provided.
Adjust unit-tests as we no longer raise an exception if there is a meeting at the said time/date
Merge pull request #64 from fedora-infra/remove_check_busy

Remove check busy