Try if notifiable has notifiable methods

Why:

There are Notifications with associated `notifiables` that actually are
not anymore Notifiables (the class doesn't include the Notifiable
concern). So when Notification delegates certain "notifiable" methods
to them the is an error.

How:

Using `try` directly on the notifiable association to avoid the delegate
trap on those corner case scenarios.

app/views/notifications/_notification.html.erb

@@ -1,5 +1,5 @@
 <li id="<%= dom_id(notification) %>" class="notification <%= 'unread' if notification&.unread? %>">
-  <% if notification.try(:notifiable_available?) %>
+  <% if notification.notifiable.try(:notifiable_available?) %>
     <% locals = { notification: notification,
                   timestamp: notification.timestamp,
                   title: notification.notifiable_title,