Add admin link in top navbar.

app/views/layouts/_top_navigation.html.erb

@@ -29,6 +29,11 @@
       </li>
     </ul>
     <form class="form-inline my-2 my-lg-0">
+      <ul class="navbar-nav mr-auto">
+        <li class="nav-item">
+          <%= link_to "Admin", rails_admin_path, class: 'nav-link' %>
+        </li>
+      </ul>
       <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
       <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
     </form>