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

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

layouts/partials/serieslinkplaceholder.html

@@ -0,0 +1,10 @@
+<div class="{{ .direction }} wrapper">
+  <div class="ongoing-placeholder">
+    {{ if eq .direction "previous" }}{{ partial "icon.html" "chevrons-left" }}{{ end }}
+    <div class="title-subtitle">
+      {{ title .direction }} in Series
+      <div class="title">Coming soon!</div>
+    </div>
+    {{ if eq .direction "next" }}{{ partial "icon.html" "chevrons-right" }}{{ end }}
+  </a>
+</div>