R1:ab4324148af7

Make the Aphlict server more resilient.

Summary:
Currently, the Aphlict server will crash if invalid JSON data is `POST`ed to it. I have fixed this to, instead, return a 400. Also made some minor formatting changes.

Ref T4324. Ref T5284. Also, modify the data structure that is passed around (i.e. `POST`ed to the Aphlict server and broadcast to the Aphlict clients) to include the subscribers. Initially, I figured that we shouldn't expose this information to the clients... however, it is necessary for…