Why this change?
On a slow network, using the `AceEditor` component will result in a blob
of text being shown first before being swapped out with the `ace.js`
editor after it has completed loading.
There is also a problem when setting the theme for the editor which
would result in a "flash" as reported in
https://github.com/ajaxorg/ace/issues/3286. To avoid this, we need to
load the theme js file before displaying the editor.
What does this change do?
1. Adds a loading spinner and set the `div.ace` with a `.hidden` class.
2. Once all the relevant scripts and initialization is done, we will
then remove the loading spinner and remove `div.ace`.
