Add a placeholder for "next in series" if the series is ongoing

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

assets/scss/style.scss

@@ -381,7 +381,7 @@ figure {
     }
 
 
-    a {
+    a, .ongoing-placeholder {
         @include bordered-block;
         padding: 0.5em 1em 0.5em 1em;
         font-size: 1.1em;
@@ -398,6 +398,10 @@ figure {
         }
     }
 
+    .ongoing-placeholder {
+      border-style: dashed;
+    }
+
     div.title {
         font-weight: bold;
     }