Use adaptive CDN TTLs for page views

Use adaptive CDN TTLs for page views

This makes frequently changed pages be less likely to be
seen in stale forms if purges are delayed or lost. Pages
that have not been edited for somewhat longer than the
nominal CDN cache TTL will retain the nominal TTL.

Category pages will adapt based on page_touched rather
than the last revision, given the possibility of constaintly
changing membership. With their lesser overall usage,
this is less risky than for Article, and also more useful.

Change-Id: If621aca2fb68e9f87a50c891dac8dc6ec7641f5c