make ckan cacheable

[toby]

need to sort cookies etc

should work with nginx and varnish

[toby]

issues:

1) want to cache static files long term - need to add a ?ver=xxx to allow changes - this will need a change to all templates etc - could be seen as a good thing

2) we can only purge specific pages not whole cache so we need to know all the urls which want to be invalidated - including language variants

3) urls should be consistent so we should use id not name for datasets etc

4) for now we can just use short expiry times as this prevents some of these problems

[toby]

Implementation of this has been carried out at the page level.

ckan.page_cache_enabled is config option to enable it

requires redis

need to document it somewhere