feat(media): add external media library support, Uploadcare integration (#1602)

packages/netlify-cms-lib-util/src/loadScript.js

@@ -0,0 +1,24 @@
+/**
+ * Simple script loader that returns a promise.
+ */
+export default function loadScript(url) {
+  return new Promise((resolve, reject) => {
+    let done = false;
+    const head = document.getElementsByTagName('head')[0];
+    const script = document.createElement('script');
+    script.src = url;
+    script.onload = script.onreadystatechange = function() {
+      if (
+        !done &&
+        (!this.readyState || this.readyState === 'loaded' || this.readyState === 'complete')
+      ) {
+        done = true;
+        resolve();
+      } else {
+        reject();
+      }
+    };
+    script.onerror = error => reject(error);
+    head.appendChild(script);
+  });
+}