LPS-46221 AutoComplete Plugin for CKEditor

Hey @natecavanaugh,

This PR adds an autocomplete plugin similar to the one we have for mentions in textareas.

You can test it [using this PR](https://github.com/natecavanaugh/liferay-plugins/pull/244) I've sent you to your plugins repo and deploying the mentions portlet. After that, if you have users, typing `@` inside an editor should trigger the autocomplete request and show the list with the matching users.

**For this to work in most places in control panel, we'll need to bring this [fix](https://issues.liferay.com/browse/AUI-1290) from AlloyUI**

Let us know if you see any issues.

Thanks!

cc/ @ipeychev