feat: add waiting page

src/epub2go_web/static/styles.css

@@ -48,6 +48,9 @@ small{
     border-color: var(--bg-acc);
     box-shadow: var(--bg-acc) 2px 2px;
 }
+.main-banner{
+    text-align: center;
+}
 table, tr{
     /* make table not resize when elements are hidden by searching */
     width: 100%;

src/epub2go_web/templates/waiting.html

@@ -0,0 +1,6 @@
+{% extends 'index.html' %}
+{% block main %}
+<main>
+    <p class="main-banner">Einen Moment bitte. Deine Datei wird heruntergeladen.</p>
+</main>
+{% endblock %}