Fix dimensions of loading component for compose drawer (#4872)

app/javascript/mastodon/features/ui/components/drawer_loading.js

@@ -0,0 +1,11 @@
+import React from 'react';
+
+const DrawerLoading = () => (
+  <div className='drawer'>
+    <div className='drawer__pager'>
+      <div className='drawer__inner' />
+    </div>
+  </div>
+);
+
+export default DrawerLoading;