Poll every 30 seconds for updates made to the currently open
change. If a modification is detected show a small butter
bar in the bottom right corner advising the user they are
viewing a stale version of the change.
To be efficient this relies on the recently added support to
reply "304 Not Modified" when there are no updates.
change.updateDelay can be configured by the site administrator
to manage the polling frequency.
Change-Id: Ib3f3acf513193bf1f1f92c472cd586999f3cad5d
b9ebb668f6