Normalize trailing slashes in the page cache

Probably the main issue with this is it might prevent redirect/global redirect from always redirecting to the trailing forward slash version of the path - previously that redirect would have prevented page caching from writing to the cache due to the redirect.

Maybe if redirect runs in an early enough middleware it might not though, or we could open an issue about it?

Seems to caused a cache test failure in blocks.

RedirectLeadingSlashesSubscriber adds a cacheable redirect if there are multiple leading slashes, should we just extend this to trailing slashes?