Add NotFound handler (#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>

routers/web/web.go

@@ -1078,4 +1078,14 @@ func RegisterRoutes(m *web.Route) {
 	if setting.API.EnableSwagger {
 		m.Get("/swagger.v1.json", SwaggerV1Json)
 	}
+	m.NotFound(func(w http.ResponseWriter, req *http.Request) {
+		escapedPath := req.URL.EscapedPath()
+		if len(escapedPath) > 1 && escapedPath[len(escapedPath)-1] == '/' {
+			http.Redirect(w, req, setting.AppSubURL+escapedPath[:len(escapedPath)-1], http.StatusTemporaryRedirect)
+			return
+		}
+		ctx := context.GetContext(req)
+		ctx.NotFound("", nil)
+	})
+
 }