Add NotFound handler

PR go-gitea#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 go-gitea#18060

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

integrations/links_test.go

@@ -49,6 +49,7 @@ func TestRedirectsNoLogin(t *testing.T) {
 	defer prepareTestEnv(t)()
 
 	var redirects = map[string]string{
+		"/user2/repo1/":                              "/user2/repo1",
 		"/user2/repo1/commits/master":                "/user2/repo1/commits/branch/master",
 		"/user2/repo1/src/master":                    "/user2/repo1/src/branch/master",
 		"/user2/repo1/src/master/file.txt":           "/user2/repo1/src/branch/master/file.txt",

routers/web/web.go

@@ -1078,4 +1078,13 @@ 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.StatusFound)
+		}
+		ctx := context.GetContext(req)
+		ctx.NotFound("", nil)
+	})
+
 }