Move to our own SSE implementation

[rodrigo-o]

Implementing a quick SSE stream function shouldn't be a really complex issue, we are right now using sse and event_bus libraries for convenience due to they working pretty much out of the box, but the idea is to migrate to phoenix_pubsub and implement the SSE event listener on our own. For now the current implementation works.

The more important functions to take a look at are Plug.Conn.send_chunked and Plug.Conn.chunk