Rewrite trailing slashes to canonical URL

If user hits https://www.gov.uk/browse/driving/, it redirects to https://www.gov.uk/browse/driving and does load the correct page.

Currently on mirror, we get a 503 in above scenario, because the redirection is usually handled by the application.

This commit adds a rewrite which will do that redirection for all URLs ending in a slash - I've confirmed that we don't have any URLs which should end in a trailing slash, so this change will make the mirror match the behaviour of the GOV.UK Stack.

This [Fixes #56337502] once it has been deployed