Implement turn-based conversation modalities

[cjqian]

Features needed to make this happen:

• P0: Experimenters can check a "turn-based" (as opposed to default free-form) conversation modality
• P0: Participants are randomly sampled in order (?) and can speak when it is their turn (a "It's your turn to speak!" and others are disabled); other chat fields are disabled when it's not their turn
• P1: Design decision: Can AI mediators speak whenever?
• P1: Make sure agent / AI participants work with this feature
• P1: Make sure this doesn't break API features (maybe get rasmi@ to take a look at incoming PR)
• P2: Update the documentation to reflect this feature