Don't update past meeting

If someone has the direct link to a past meeting, they can re-schedule/update the meeting. The link will always be present on the GH issue and in the calendar invit which makes this a possibility.
The tool should not allow any edits/updates if the meeting time has passed.

Find the entry point that handles meeting updates and inspect how the meeting time is obtained. Reproduce the direct-link case, then add coverage showing that past meetings cannot be edited while current or future meeting updates remain allowed.

Written by the indexing model from the issue text.