Add cache troubleshooting for emergency banner
Add cache troubleshooting for emergency banner

The emergency banner tests often result in pages not showing the banner
or not clearing the banner despite the use of cache-bust strings or
origin cache-clearing. Add some hints and tips for developers. In
particular, remind us to test on the appropriate environment
(staging/integration) and discount any browser based "from memory"
caching (hello Chrome) by using curl. Also note the possibility that the
caching layers on GOV.UK might have evolved, requiring changes to the
fab scripts that clear caches.

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