Add a setting to retrieve bookmark metadata asynchrounously

- There is a new standalone script (metadata.js) which requests
    a new controller to get bookmark metadata and fill the form async
  - This feature is enabled with the new setting: general.enable_async_metadata
    (enabled by default)
  - general.retrieve_description is now enabled by default
  - A small rotating loader animation has a been added to bookmark inputs
    when metadata is being retrieved (default template)
  - Custom JS htmlentities has been removed and  mathiasbynens/he
    library is used instead

Fixes #1563

assets/common/js/metadata.js

@@ -0,0 +1,39 @@
+import he from 'he';
+
+function clearLoaders(loaders) {
+  if (loaders != null && loaders.length > 0) {
+    [...loaders].forEach((loader) => {
+      loader.classList.remove('loading-input');
+    });
+  }
+}
+
+(() => {
+  const loaders = document.querySelectorAll('.loading-input');
+  const inputTitle = document.querySelector('input[name="lf_title"]');
+  if (inputTitle != null && inputTitle.value.length > 0) {
+    clearLoaders(loaders);
+    return;
+  }
+
+  const url = document.querySelector('input[name="lf_url"]').value;
+  const basePath = document.querySelector('input[name="js_base_path"]').value;
+
+  const xhr = new XMLHttpRequest();
+  xhr.open('GET', `${basePath}/admin/metadata?url=${encodeURI(url)}`, true);
+  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
+  xhr.onload = () => {
+    const result = JSON.parse(xhr.response);
+    Object.keys(result).forEach((key) => {
+      if (result[key] !== null && result[key].length) {
+        const element = document.querySelector(`input[name="lf_${key}"], textarea[name="lf_${key}"]`);
+        if (element != null && element.value.length === 0) {
+          element.value = he.decode(result[key]);
+        }
+      }
+    });
+    clearLoaders(loaders);
+  };
+
+  xhr.send();
+})();