Add GitHub Ribbon to our lessons

Backport of 9a19e91 from swcarpentry/site.

All credits to Abigail Cabunoc <abigail.cabunoc@oicr.on.ca>
for the awesome work with CSS.

swc-bootstrap.css

@@ -69,3 +69,80 @@ code {
   border-top-color: #ffffff;
   border-bottom-color: #ffffff;
 }
+
+/* GitHub Ribbon */
+
+#github-ribbon a {
+  background:#000;
+  color:#fff;
+  text-decoration:none;
+  font-family:arial, sans-serif;
+  text-align:center;
+  font-weight:bold;
+  padding:5px 40%;
+  font-size:0.8rem;
+  line-height:1.6rem;
+  position:relative;
+  transition:0.5s;
+  width:100%;
+  margin: 0 auto;
+  white-space: nowrap;
+  z-index:10;
+}
+
+#github-ribbon a:hover {
+  background:#600;
+  color:#fff;
+}
+
+#github-ribbon a::before, #github-ribbon a::after {
+  content:"";
+  width:100%;
+  display:block;
+  position:absolute;
+  top:1px;
+  left:0;
+  height:1px;
+  background:#fff;
+}
+
+#github-ribbon a::after{
+  bottom:1px;
+  top:auto;
+}
+
+/* github ribbon breaking point */
+@media screen and (min-width:600px){
+  #github-ribbon{
+    position:absolute;
+    display:block;
+    top:0;
+    right:0;
+    width:150px;
+    overflow:hidden;
+    height:150px;
+  }
+  #github-ribbon a{
+    width:200px;
+    position:absolute;
+    padding:5px 40px;
+    top:40px;
+    right:-80px;
+    transform:rotate(45deg);
+    -webkit-transform:rotate(45deg);
+    box-shadow:1px 1px 10px rgba(0,0,0,0.8);
+  }
+  .navbar .btn-navbar {
+    margin-right: 150px;
+  }
+}
+
+@media (max-width: 319px) {
+  .header h1 {
+      font-size: 20pt;
+  }
+
+  #header-text {
+      font-size:16pt;
+  }
+}