Move event signing to server-server spec

It's *definitely* not a client concern.

It isn't really in the right place at the moment, but s-s is very much a
work in progress whereas c-s is nearly stable.

event-schemas/schema/core-event-schema/room_event.json

@@ -21,6 +21,7 @@
     "unsigned": {
       "type": "object",
       "description": "Contains optional extra information about the event.",
+      "title": "UnsignedData",
       "properties": {
         "age": {
           "type": "integer",