feat(breakout-rooms): enforce assignment of every participant in manual mode

Maximilian Fußrequested to merge
m.fuss/web-app:3536-backport-breakout-rooms-enforce-assignment-of-every-participant-to-v2-9-x into release/v2.9.x

feat(breakout-rooms): enforce assignment of every participant in manual mode

Closes #3536 (closed)

(cherry picked from commit 55685aca)

Closes #3536 (closed)

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