Make m.presence match what synapse returns

Considering this is the behaviour that is embedded into most client libraries and the most popular homeserver, it should be adopted as the standard.

Signed-off-by: Travis Ralston <travpc@gmail.com>

event-schemas/examples/m.presence

@@ -4,8 +4,7 @@
     "last_active_ago": 2478593,
     "presence": "online",
     "currently_active": false,
-    "user_id": "@example:localhost"
   },
-  "event_id": "$WLGTSEFSEF:localhost",
+  "sender": "@example:localhost",
   "type": "m.presence"
 }

event-schemas/schema/m.presence

@@ -29,21 +29,17 @@
                 "currently_active": {
                     "type": boolean,
                     "description": "Whether the user is currently active"
-                },
-                "user_id": {
-                    "type": "string",
-                    "description": "The user's ID."
                 }
             },
-            "required": ["presence", "user_id"]
+            "required": ["presence"]
         },
         "type": {
             "type": "string",
             "enum": ["m.presence"]
         },
-        "event_id": {
+        "sender": {
             "type": "string"
         }
     },
-    "required": ["event_id", "type", "content"]
+    "required": ["sender", "type", "content"]
 }