Link to Fahrplan from event page

app/views/sessions/show.html.erb

 <div>
   <h6><%= link_to @session.conference.name, @session.conference %></h6>
-  <h1><%= @session.title %></h1>
+  <h1>
+    <%= @session.title %>
+    <% unless @session.url.blank? %>
+    <%= link_to "🔗 open in Fahrplan", @session.url, class: "ml-4 font-normal" %>
+    <% end %>
+  </h1>
   <h2><%= @session.language %> <%= @session.is_interpreted ? "(live interpretation)" : "" %></h2>
   <h2><%= @session.starts_at.strftime("%Y-%m-%d") %> <%= @session.starts_at.strftime("%H:%M") %> &ndash; <%= @session.ends_at.strftime("%H:%M") %></h2>
   <h2><%= @session.stage.name %>&middot; <%= @session.session_format %> &middot; <%= @session.track %></h2>