improve online meeting experience for members/wgs

this is a followup to #19

As per discussions and feedback around we would like to solve the following

1) conflict schedule: we're growing, running out of room for meeting times that do not conflict between WGs
2) we have some issues to identify the events on the schedule per wg. We should also manage who can be an host.

Other features that could be added:

* transcriptions
* allows people external to a WG to join a meeting if this meeting is open: maintain a calendar?

Possible solutions:

1) As suggested in #19 we can upgrade to another level of Zoom and asked for a non-profit discount
2) An alternative would be using an opensource and free self-hosted solution. We tested Jitsi on the platform maintained by the french organisation Erlang Connect which worked well. Other solutions are possible like BigBlueButton.

Let list Pro/Cons of solutions in comments. If you see another alternative please add a a comment. Would be nice also to propose a way to test it.