Move plugin version menu dependencies to script (vagrant-libvirt#1545)

Move to manage the entire plugin version menu and dependencies from the
same script. This will allow the entire site to roll forward to newer
releases and bugfix the plugin versions menu as issues are found, mostly
without needing to touch any of the individual releases.

docs/_includes/head_custom.html

@@ -1,8 +1,2 @@
-<script src="https://cdnjs.cloudflare.com/ajax/libs/axios/0.27.2/axios.min.js"></script>
-<script
-  src="https://cdn.jsdelivr.net/npm/[email protected]/dist/index.bundle.js"
-  integrity="sha256-yJbSlTxKmgU+sjlMx48OSjoiUsboy18gXTxUBniEEO0="
-  crossorigin="anonymous"
-></script>
 <link rel="stylesheet" href="{{ '/assets/css/just-the-docs-light.css' | relative_url }}" media="(prefers-color-scheme: light)">
 <link rel="stylesheet" href="{{ '/assets/css/just-the-docs-dark.css' | relative_url }}" media="(prefers-color-scheme: dark)">

docs/assets/js/plugin_versions_menu.js

@@ -1,8 +1,61 @@
-const { buildWebStorage, setupCache } = window.AxiosCacheInterceptor;
-const storage = buildWebStorage(sessionStorage, 'axios-cache:');
-const axiosCached = setupCache(axios.create(), { storage });
+// Look at webpack to replace the dependency loading
+// also would allow use yarn + jest to support unit testing code below
+
+const dependencies = [
+    {
+        src: 'https://cdnjs.cloudflare.com/ajax/libs/axios/0.27.2/axios.min.js',
+        integrity: 'sha512-odNmoc1XJy5x1TMVMdC7EMs3IVdItLPlCeL5vSUPN2llYKMJ2eByTTAIiiuqLg+GdNr9hF6z81p27DArRFKT7A==',
+        crossOrigin: 'anonymous',
+    },
+    {
+        src: 'https://cdn.jsdelivr.net/npm/[email protected]/dist/index.bundle.js',
+        integrity: 'sha256-yJbSlTxKmgU+sjlMx48OSjoiUsboy18gXTxUBniEEO0=',
+        crossOrigin: 'anonymous',
+    },
+]
+
+const loaded = [];
+
+dependencies.forEach( (dep, i) => {
+    loaded[i] = false;
+    loadScript(dep, function() {
+        loaded[i] = true;
+        if (loaded.every(value => value === true)) {
+            // arguments come from a site constants file
+            handleVersionedDocs(repository_nwo, basePath);
+        }
+    });
+})
+
+// helper taken from https://stackoverflow.com/a/950146 under CC BY-SA 4.0, added support
+// for additional attributes to be set on the script tag.
+function loadScript(url, callback)
+{
+    // Adding the script tag to the head as suggested before
+    var head = document.head;
+    var script = document.createElement('script');
+    script.type = 'text/javascript';
+    script.src = url.src;
+    delete url.src;
+    for (var attribute in url) {
+        script[attribute] = url[attribute];
+    }
+
+    // Then bind the event to the callback function.
+    // There are several events for cross browser compatibility.
+    script.onreadystatechange = callback;
+    script.onload = callback;
+
+    // Fire the loading
+    head.appendChild(script);
+}
+
+// main function, must wait until dependencies are loaded before calling
+function handleVersionedDocs(repository_nwo, basePath) {
+    const { buildWebStorage, setupCache } = window.AxiosCacheInterceptor;
+    const storage = buildWebStorage(sessionStorage, 'axios-cache:');
+    const axiosCached = setupCache(axios.create(), { storage });
 
-changeVersion = function handleVersionedDocs(repository_nwo, basePath) {
     menuBackgroundImageClosed = "url(\"data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='currentColor' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpolyline points='15 6 9 12 15 18'%3E%3C/polyline%3E%3C/svg%3E\")";
     menuBackgroundImageOpen = "url(\"data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='currentColor' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpolyline points='6 9 12 15 18 9'%3E%3C/polyline%3E%3C/svg%3E\")";
 
@@ -173,4 +226,4 @@ changeVersion = function handleVersionedDocs(repository_nwo, basePath) {
             hideMenu();
         }
     });
-}(repository_nwo, basePath);
+}