Document the icon package used

[Aiky30]

Currently it' impossible to know where the icons came from in the project, to fix this the following information should be documented:

The icons are taken from Font Awesome 4. They can be found at the following url: https://github.com/FortAwesome/Font-Awesome/tree/v4.7.0

Examples of the icons to help choose: https://fontawesome.com/v4.7/icons/

The SVG's have been taken from here: https://github.com/Rush/Font-Awesome-SVG-PNG/tree/master/black/svg

[fsbraun]

We might want to consider putting the most common icons into the icon font django-cms core generates. That will be more efficient and allow for much easier and uniform changes in style since the icon color is not hard coded into the font.

Right now djangocms-admin-style pins the color scheme to light mode. Bare django admin will have an issue with dark icons in dark mode.

[fsbraun]

As of #302 versioning uses the icons of the core.