Implement basic navbar

Signed-off-by: Tobias Eidelpes <tobias@eidelpes.info>

app/assets/stylesheets/application.css

@@ -13,3 +13,66 @@
  *= require_tree .
  *= require_self
  */
+
+body {
+    margin: 0;
+}
+
+main {
+    margin: 8px;
+}
+
+/* Add a black background color to the top navigation */
+.topnav {
+    background-color: #333;
+    overflow: hidden;
+    box-shadow: 0 16px 24px 2px rgba(0,0,0,0.14), 0 6px 30px 5px rgba(0,0,0,0.12), 0 8px 10px -5px rgba(0,0,0,0.20);
+}
+
+/* Style the links inside the navigation bar */
+.topnav a {
+    float: left;
+    display: block;
+    color: #f2f2f2;
+    text-align: center;
+    padding: 18px 20px;
+    text-decoration: none;
+    font-size: 17px;
+}
+
+/* Change the color of links on hover */
+.topnav a:hover {
+    background-color: #ddd;
+    color: black;
+}
+
+/* Add an active class to highlight the current page */
+.topnav a.active {
+    background-color: #4CAF50;
+    color: white;
+}
+
+.topnav a:not(.active) {
+    display: none;
+}
+
+.topnav a.icon {
+    float: right;
+    display: block;
+    padding: 14px 16px;
+}
+
+/* Layout for responsive class; gets added on menu click */
+.topnav.responsive {
+    position: relative;
+}
+.topnav.responsive a.icon {
+    position: absolute;
+    right: 0;
+    top: 0;
+}
+.topnav.responsive a {
+    float: none;
+    display: block;
+    text-align: left;
+}