Prevent scroll when modal is up.

undercover/static/styles.css

@@ -417,4 +417,8 @@ textarea {
 
 .hidden {
   display: none;
+}
+
+.scroll-lock {
+  overflow: hidden;
 }

undercover/templates/base.jinja2

@@ -9,10 +9,12 @@
     <script type="text/javascript">
       function closeModal() {
         document.getElementById('modal').classList.add('transparent')
+        document.body.classList.remove('scroll-lock')
       }
       function showModal(login, text) {
         document.getElementById('modal').classList.remove('transparent')
         document.getElementById('modal-title').innerText = text
+        document.body.classList.add('scroll-lock')
         const createAccountElements = [
           'confirm-password',
           'confirm-password-label',