Move old version notice into template with .hidden

doc/src/hexdoc/_templates/components/welcome.html.jinja

@@ -17,8 +17,10 @@
       progression. Alternatively, click <a href="?nospoiler">here</a> to get a version with all spoilers showing.
     </p>
 
-    {% if not is_bleeding_edge %}
-      <div id="old-version-notice"></div>
-    {% endif %}
+    {# conditionally revealed using js #}
+    <span id="old-version-notice" class="hidden">
+      <br />
+      <i>The past is a foreign country; they do things differently there.</i>
+    </span>
   </blockquote>
 </div>

doc/src/hexdoc/_templates/main.css

@@ -109,6 +109,11 @@ canvas.spell-viz {
   white-space: nowrap;
 }
 
+.hidden {
+  display: none;
+  visibility: hidden;
+}
+
 @media (min-width: 40rem) {
   .book-body {
     width: 70%;

doc/src/hexdoc/_templates/main.js.jinja

@@ -370,17 +370,10 @@ function sortSitemapVersions(sitemap) {
 function addDropdowns(sitemap) {
   let [branches, versions] = sortSitemapVersions(sitemap);
 
-  // set the "old version" message if this page is a version number, but not the latest one
+  // reveal the "old version" message if this page is a version number, but not the latest one
   // this isn't a dropdown, but it's here since we have the data anyway
   if (versions.slice(1).includes(VERSION)) {
-    // thanks for the idea petra
-    let oldVersionNotice = document.createElement("i");
-    oldVersionNotice.textContent = "The past is a foreign country; they do things differently there.";
-
-    document.getElementById("old-version-notice").append(
-      document.createElement("br"),
-      oldVersionNotice,
-    );
+    document.getElementById("old-version-notice").classList.remove("hidden")
   }
 
   // versions