Add some JavaScript to open an close the mobile menu

Fixes #75 but still works in Firefox mobile without JS (and it will in Chrome/Safari too
once :focus-within will be implemented there)

static/js/menu.js

@@ -0,0 +1,13 @@
+// It should normally be working fine even without this code
+// But :focus-within is not yet supported by Webkit/Blink
+
+const button = document.getElementById('menu')
+const menu = document.getElementById('content')
+
+button.addEventListener('click', () => {
+  menu.classList.add('show')
+})
+
+menu.addEventListener('click', () => {
+  menu.classList.remove('show')
+})