Remove dependency from FA's css

[why]
Keep the dependencies as small as possible.
We already have the github icon in our own set which we import anyhow,
so there is no need to fetch fontawesome's CSS.

[how]
Replace fa-github with nf-fa-github.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

_includes/css/nerd-font-tweaks.scss

@@ -535,10 +535,8 @@ a.nerd-font-button:before {
 	}
 }
 */
+
 // Github url button
-
-@import url("https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css");
-
 #github {
 	text-decoration: none;
 	color: #000000;

index.html

@@ -7,7 +7,7 @@
   <div id="main">
     <div class="container">
       <a href="https://github.com/ryanoasis/nerd-fonts/tree/master" target="_blank" id="github"
-        class="fa fa-github"></a>
+        class="nf nf-fa-github"></a>
     </div>
     {% include topnav.html %}
     {% for page in site.posts reversed %}