Lift restriction about adding meeting in the past

This will be required for allowing someone to upload an ICS file which may contain
meetings that have already occured.

Fixes https://fedorahosted.org/fedocal/ticket/76
Adjust unit-test as we no longer raise an exception for adding/editing meeting in the past
Merge pull request #67 from fedora-infra/fix_adding_past_meeting

Fix adding past meeting