card header fixes, linkedin color

src/scss/_variables.scss

@@ -151,6 +151,7 @@ $code-color: $primary !default;
 $social-colors: (
   "facebook": #3b5998,
   "twitter": #1da1f2,
+  "linkedin": #0a66c2,
   "google": #dc4e41,
   "youtube": #ff0000,
   "vimeo": #1ab7ea,

src/scss/ui/_cards.scss

@@ -217,6 +217,10 @@ Card footer
   margin-top: -$card-spacer-y;
   margin-bottom: $card-spacer-y;
   color: $text-muted;
+
+  .card-header & {
+    margin: -.125rem 0 0;
+  }
 }
 
 .card-body {