Cache RecentChanges Atom feed in varnish for 15 seconds

Prior to fc20c30d209, the feed was cached for 10 seconds. That
change accidentally removed varnish caching. Re-add the caching,
and upping to 15 seconds for good measure (15 second delay on the RSS
feed shouldn't matter, people generally read those asynchronously
anyhow. I was thinking of maybe even uping to 30 seconds). Users
can still manually set an smaxage parameter via the url if they
don't like the default cache time.

This assumes nothing private (e.g. per user) is ever on the RSS
feed. Reading through the ChangesList code this appears to be
the case, as everyone is getting the feed from a shared
memcache entry.

Bug: 63249
Change-Id: I268d0a53067738ba96bee74c593358b0b28cc083