Update from upstream

Include language in page cache key, the same as is in v3.11

Added option to extend page_cache_key by referencing a function in settings.CMS_PAGE_CACHE_KEY_EXTRA as string formatted for import_module.

This is needed when language is moved away from url path, it needs to be provided separately, otherwise cache is mixed.

This is also useful if request contain any other cache differentiators, for example currency in a cookie.

Added option to extend get_placeholder_cache_key by referencing a function in settings.CMS_PLACEHOLDER_CACHE_KEY_EXTRA as string formatted for import_module.

This is needed when some session setting change page content (e.g. currency), otherwise cache is mixed.