when user opens url to admin or account endpoint, but without trailing slash, redirect them to the url with trailing slash

the trailing slash is commonly forgotten. in the default setup, for the admin
endpoint, this makes you end up at the account endpoint, which won't accept
your admin credentials. with this change, users won't get confused by that
anymore.

for issue #43

http/web.go

@@ -328,6 +328,16 @@ func (s *serve) ServeHTTP(xw http.ResponseWriter, r *http.Request) {
 // Listen binds to sockets for HTTP listeners, including those required for ACME to
 // generate TLS certificates. It stores the listeners so Serve can start serving them.
 func Listen() {
+	redirectToTrailingSlash := func(srv *serve, name, path string) {
+		// Helpfully redirect user to version with ending slash.
+		if path != "/" && strings.HasSuffix(path, "/") {
+			handler := safeHeaders(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				http.Redirect(w, r, path, http.StatusSeeOther)
+			}))
+			srv.Handle(name, nil, path[:len(path)-1], handler)
+		}
+	}
+
 	for name, l := range mox.Conf.Static.Listeners {
 		portServe := map[int]*serve{}
 
@@ -365,6 +375,7 @@ func Listen() {
 			srv := ensureServe(false, port, "account-http at "+path)
 			handler := safeHeaders(http.StripPrefix(path[:len(path)-1], http.HandlerFunc(accountHandle)))
 			srv.Handle("account", nil, path, handler)
+			redirectToTrailingSlash(srv, "account", path)
 		}
 		if l.AccountHTTPS.Enabled {
 			port := config.Port(l.AccountHTTPS.Port, 443)
@@ -375,6 +386,7 @@ func Listen() {
 			srv := ensureServe(true, port, "account-https at "+path)
 			handler := safeHeaders(http.StripPrefix(path[:len(path)-1], http.HandlerFunc(accountHandle)))
 			srv.Handle("account", nil, path, handler)
+			redirectToTrailingSlash(srv, "account", path)
 		}
 
 		if l.AdminHTTP.Enabled {
@@ -386,6 +398,7 @@ func Listen() {
 			srv := ensureServe(false, port, "admin-http at "+path)
 			handler := safeHeaders(http.StripPrefix(path[:len(path)-1], http.HandlerFunc(adminHandle)))
 			srv.Handle("admin", nil, path, handler)
+			redirectToTrailingSlash(srv, "admin", path)
 		}
 		if l.AdminHTTPS.Enabled {
 			port := config.Port(l.AdminHTTPS.Port, 443)
@@ -396,6 +409,7 @@ func Listen() {
 			srv := ensureServe(true, port, "admin-https at "+path)
 			handler := safeHeaders(http.StripPrefix(path[:len(path)-1], http.HandlerFunc(adminHandle)))
 			srv.Handle("admin", nil, path, handler)
+			redirectToTrailingSlash(srv, "admin", path)
 		}
 		if l.MetricsHTTP.Enabled {
 			port := config.Port(l.MetricsHTTP.Port, 8010)