For support room invites, fix multiple-role people not being invited

[reivilibre]

Pulled out of FOSDEM 2024 hotfixes #228

Two things here:

• in the general case, we make it so that the Coordinator trumps Speaker. This at least makes it a little more deterministic when there's a conflict, but I think we ought to consider supporting multiple role people a bit more natively/robustly in the future. (but at short-notice, this would have been too error-prone to do)
• for the support room invite case, we fix dual-role wielders not being invited to the right room, by applying the correct filtering at the source to avoid role conflicts messing stuff up.