(CE-669) Add some client-side caching of anon SWM requests

Add 15 minute client-side caching of anon SWM checks and make use of
the last dismissed message ID to bust the cache when a message is
dismissed by the user.
(CE-669) Change Varnish and Browser TTL
Merge pull request #2950 from Wikia/CE-669

(CE-669) Add some client-side caching of anon SWM requests