message_store: Add an each helper.

This new helper allows us to do the same operation
on every message in our message_store.  We will
use this in a future commit to clear the `is_tall`
flags on all messages, after a resize.

We should be somewhat cautious about using this,
but simple operations should be really fast, even
if you have lots of messages in the store.

frontend_tests/node_tests/message_store.js

@@ -184,6 +184,12 @@ run_test('update_booleans', () => {
     assert.equal(message.unread, true);
 });
 
+run_test('each', () => {
+    message_store.each((message) => {
+        assert(message.alerted !== undefined);
+    });
+});
+
 run_test('message_id_change', () => {
     var message = {
         sender_email: 'me@example.com',