Remind the user if they're trying to reply to someone who has left

This happens semi-frequently to me and others - we reply to a user who
has left the room. Sometimes this is useful (for example, bringing up a
previous topic) but in most cases this is accidental, and almost
guaranteed to happen if you turned off join/leave events.

So I added a reminder when replying - which manifests as a small label
in the chatbar - that the user has left and can't be notified of your
reply.

autotests/data/test-min-sync-extra-sync.json

@@ -0,0 +1,20 @@
+{
+  "state": {
+    "events": [
+      {
+        "content": {
+          "membership": "leave"
+        },
+        "event_id": "$1432735824666PhrSA:example.org",
+        "origin_server_ts": 1432735824666,
+        "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+        "sender": "@example:example.org",
+        "state_key": "@example:example.org",
+        "type": "m.room.member",
+        "unsigned": {
+          "replaces_state": "$143273582443PhrSn:example.org"
+        }
+      }
+    ]
+  }
+}