Replace socket.io with plain WebSocket

Replace socket.io with plain WebSocket using the ws library as it
doesn't really provide any useful tools over plain WebSocket when it
comes to implementing reliable communication that works across
reconnects.  The widespread browser support for WebSocket also means
it has little value to add for the web interface over WebSocket.

docs/socket.md

@@ -5,21 +5,9 @@ Socket.io protocol
 slaves, and unless you're developing Clusterio it's probably not going
 to be very useful for you.</sub>
 
-The protocol is built on the [socket.io](https://socket.io/) library
-which lets mulitple streams be multiplexed in many different ways over
-the same connection and abstracts away reconnection logic.
-Unfortunately I've been unable to figure what the message delivery
-guarantees of the library is, so whether or not messages can be lost is
-still an unknown, I assume that messages are delivered as long as a
-connection is maintained or manages to reconnect.
-
-While there are many mechanisms to multiplex streams, the core
-communication between the master server and slaves is done entirely by
-sending single POJO payloads over the `message` event.  There's a
-shortcut for sending these via the `.send` method of the socket.io
-sockets.
-
-These messages contain the following three properties:
+The protocol is built on WebSocket and uses JSON encoded text messages
+to communicate to and from the master server.  These messages contain
+the following three properties:
 - seq - integer - The sequence number of the message
 - type - string - The type of message
 - data - object - Message data
@@ -28,7 +16,7 @@ These messages contain the following three properties:
 Handshake
 ---------
 
-Upon connecting to the socket.io endpoint the server will respond with a
+Upon connecting to the WebSocket endpoint the server will respond with a
 `hello` message containing the master server version as the data.  The
 client is then expected to send a `register_slave` or `register_control`
 message back depending on what kind of client it is.  If the register