[chat_history] send messages by directly writing to firebase collection

[kevmo314]

Because Firestore is more resilient, we should directly write to it instead of passing through a function. This will have better eventual consistency semantics since firebase functions may time out due to lack of connection whereas Firestore will guarantee that the message send action is written eventually. #781 will need to be submitted to allow authenticated writes to the correct subcollection first.

[kevmo314]

We now do this.