Make rooms in MemberInfo update when necessary

Factor out the chunk of code that looks through a read receipt event
to see if it contain a read receipt from a given user, now we use it
in 2 places.

src/utils/Receipt.js

@@ -0,0 +1,32 @@
+/*
+Copyright 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * Given MatrixEvent containing receipts, return the first
+ * read receipt from the given user ID, or null if no such
+ * receipt exists.
+ */
+export function findReadReceiptFromUserId(receiptEvent, userId) {
+    var receiptKeys = Object.keys(receiptEvent.getContent());
+    for (var i = 0; i < receiptKeys.length; ++i) {
+        var rcpt = receiptEvent.getContent()[receiptKeys[i]];
+        if (rcpt['m.read'] && rcpt['m.read'][userId]) {
+            return rcpt;
+        }
+    }
+
+    return null;
+}