Add fontawesome support to oslosphinx

[Joshua Harlow]

The font awesome library @ https://fortawesome.github.io/Font-Awesome provides a large set of nice & free icons that can be used as buttons or as simple icons in the various projects documentation (for example a simple one is a email icon, or a github icon...). It would be great if the oslosphinx theme natively exposed these icons and made it easy to use them in the projects documentation.

Examples of how to do this.

1. Include the css and fonts for fontawesome in the theme.
2. Add a sphinx extension that exposes these as icons and buttons in sphinx documentation.
   - See https://github.com/jfardello/Sphinxtrap/blob/master/sphinxtrap/ext/rawtoc.py#L157 for a similar extensions

For example a example that would be nice to have working:

---
* Read over the `paradigm shifts`_ and engage the team in `IRC`_
  :btn:`<https://webchat.freenode.net/?channels=openstack-state-management> comment` (or via the
  :fonticon:`envelope` `openstack-dev`_ mailing list) if these need
  more explanation (prefix ``[TaskFlow]`` to your emails subject to get an
  even faster response).
----

The :btn: and :fonticon: would specify font awesome icons to use (as well as links and text).

I have a prototype of this working, although I'm not much of a css, sphinx expert.

[Joshua Harlow (harlowja)]

Prototype patch (hack....) that made this mostly work in taskflow.

[Doug Hellmann (doug-hellmann)]

We've deprecated this library. If we still need this feature it should be added to openstackdocstheme.