Before this commit, a meeting holding a Discuss video call link had no channel until someone opened `videocall_location`: the meeting was absent from the Discuss "Meetings" tab, and its invitation link only started working once an attendee joined the call. The one way to get a meeting out of Discuss was the "Meeting" button, which starts an ad-hoc group call with no date attached to it.

This commit gives such a meeting its channel upfront, on create and as soon as the link is added afterwards, and keeps the two in line:

- the channel is named after the meeting and describes when it takes place (the recurrence rule for a recurring meeting, its date and time otherwise), and is pinned for every attendee having a user, so the meeting shows in the "Meetings" tab of each of them;
- its avatar shows the day of the meeting rather than the day the channel was created, in the reader's own timezone;
- every occurrence of a recurrence shares a single channel, which only goes away with the last of those meetings;
- deleting a meeting takes its video call away, unless the channel already hosted a conversation: that one is only unpinned, so the chat history stays reachable from Discuss while the video call drops out of the tab.

The channel is resolved from the meeting the recurrence machinery left live, rather than from the edited record: editing a whole recurrence archives the occurrence it was asked from and moves the new values to a freshly recreated one, which would otherwise write back the values from before the edit.

The "Meetings" tab gains a "Planned" filter, keeping only the video calls backed by a meeting, and its "Meeting" button becomes a menu offering "Start Now" next to "Schedule for later", which opens a `calendar.event` form prefilled with a 30 minute Discuss video call. The button is a menu whatever the modules contributing to it, so that starting a meeting is always found in the same place. Scheduling is only offered in the webclient, as the form view it opens is not part of the public Discuss page.

Both calendar parts come from hooks defined on `MessagingMenu` itself, `extraTabActions` and `extraTabFilters`, rather than from the /discuss/ patch declaring the tab: a bundle contributing to a tab may be patched in before the one declaring it, which would then discard the override.

task-6460543




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