Moved from a static folder to a dynamically created folder in the
framework/cache directory, to increase the chance that it's created with
server-writable permissions.
This is due to an issue where users had permission issues, since adding
a new folder means it's created by the git user and often
non-web-writable.
- Updated page content cache to use app version in cache key
- Moved purifier cache into framework to better work with existing
expected folders.
- Added app version check to purifier so that it will reset its own
cache on app version change.