Add waiting room overrides for Hubble story

[Carifio24]

This PR adds the necessary functionality to enable teachers to override the waiting room in the Hubble's Law story (which will lead to the same behavior as if their class were a small class):

• Add HubbleWaitingRoomOverrides table and model
• Add endpoint hubbles_law/waiting-room-override which accepts PUT and DELETE requests to allow adding and removing this designation