When editing a meeting which has a date specified in the url, use that date

Currently it was going one meeting to far in the future because it was
looking at the next occurrence of the meeting from next week.
This is actually not the intended behavior. The function
update_date_rec_meeting is meant to do it and does it well, so let's rely
on it
Add some specific unit-tests checking the behavior of the start_date in the edit meeting form

This way we should be able to detect regression.
Check the behavior of the start_date when deleting a meeting

Since we use the ?from_date argument when deleting meetings as well we
should check for changes in behavior.
Merge pull request #159 from fedora-infra/fix_edit_date

Fix handling of ?from_date