bugfix: relax `next` verification

AUR renders its own 404 Not Found page when a bad route
is encountered. Introducing the previous verification
caused an error in this case when setting a language
while viewing the Not Found page. So, instead of checking
through routes, just make sure that the next parameter
starts with a '/' character, which removes the possibility
of any cross attacks.

+ Removed aurweb.asgi.routes; no longer needed.

Signed-off-by: Kevin Morris <kevr@0cost.org>