Merge branch 'button-icon' into 'master'

Adds a btn-icon class

See merge request gitlab-org/gitlab-ce!22870

Author: Tim Zallmann

app/assets/stylesheets/framework/buttons.scss

@@ -207,6 +207,10 @@
     @include btn-with-margin;
   }
 
+  &.btn-icon {
+    color: $gl-gray-700;
+  }
+
   .fa-caret-down,
   .fa-chevron-down {
     margin-left: 5px;