Add NotFound handler (#18062) (#18067)

Add NotFound handler (#18062) (#18067)

Backport #18062

PR #17997 means that urls with terminal '/' are no longer immediately mapped
to the url without a terminal slash. However, it has revealed that the NotFound handler
appears to have been lost.

This PR adds back in a NotFound handler that simply redirects to a path without the
terminal slash or runs the NotFound handler.

Fix #18060

Signed-off-by: Andrew Thornton <art27@cantab.net>