Emojione one is a site that is trying to standardize emoji use online. They even have a nifty developer tool where you can download the whole package for various ways to upload to your own site:
The only thing is, I'm not that familiar with xenforo still to see which is the best way of going about getting emojione on the site. They have a list of demos:
They say for installation:
<link rel="stylesheet" href="//cdn.jsdelivr.net/emojione/2.1.0/assets/css/emojione.min.css"/>
Alternatively, CDNjs is also available as a [CDN Host for Emoji One](https://cdnjs.com/libraries/emojione).