Call for sessions

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Many big conferences with a lot of parallel session use to do a call for sessions. Conveners propose sessions that might be accepted or not by the organizers. If they are accepted they are automatically added to the timetable.

The issue describes a call-for-sessions workflow in which conveners propose sessions and accepted sessions are added to the timetable. No implementation files, tests, entry points, or acceptance details are identified, so first map the existing event and timetable workflow before defining the change. Done would require an agreed design and working support for proposing, accepting, and automatically scheduling sessions.

Written by the indexing model from the issue text.