Add a twig function called icon instead of using imports / macros.

@phiamo, don't merge this yet. 

This was just to illustrate one of the ideas I had about solving this. It works pretty well, in configuration you'd just do:

    mopa_bootstrap:
        icons:
            icon_set: fontawesome  # defaults to glyphicons so no BC here

This will allow us to add more icon styles and using a template instead of just a prefix will allow other fonts that may have data attributes or something. 

Also note that I removed the `<span>` from the menu.html.twig and there should now only be a space after an icon. Otherwise there will be no space and it should be inline anyway.