mmistakes#1653: Add GitLab social icon brand color (mmistakes#1654)

Author: tlindsay42

_sass/minimal-mistakes/_utilities.scss

@@ -224,6 +224,10 @@ body:hover .visually-hidden button {
     color: $github-color;
   }
 
+  .fa-gitlab {
+    color: $gitlab-color;
+  }
+
   .fa-google-plus,
   .fa-google-plus-square,
   .fa-google-plus-g {

_sass/minimal-mistakes/_variables.scss

@@ -82,6 +82,7 @@ $facebook-color: #3b5998 !default;
 $flickr-color: #ff0084 !default;
 $foursquare-color: #0072b1 !default;
 $github-color: #171516 !default;
+$gitlab-color: #e24329 !default;
 $google-plus-color: #dd4b39 !default;
 $instagram-color: #517fa4 !default;
 $lastfm-color: #d51007 !default;