Fix selenium test flakiness

[vladimirnani]

Currently:

1. Django renders the <strong>Total messages:</strong> <span id="messageCount">0</span>
2. Test test_real_time_delete_message creates a new message and hits page refresh
3. Test refreshes the page but doesnt wait for message to be handled, so renderState is not able to update the counter
4. Test finds the existing count element and asserts that count 0 is not expected 1.

PR changes:

• explicitly wait on window.messageHandled in js to be sure that renderState has finished.

Removes flakiness on fast machine.

[carltongibson]

Ok, looks sensible. Thanks!