fix block snippet, use block.website instead of instance variable which may not be set

app/views/admin/communication/blocks/templates/agenda/_snippet.html.erb

 <% if block.template.mode == 'categories' %>
   <p>
-    <%= @website.agenda_categories
+    <%= block.website.agenda_categories
                 .map { |category| category.to_s_in(block.language) }
                 .join(', ') %>
 

app/views/admin/communication/blocks/templates/posts/_snippet.html.erb

 <%  if block.template.mode == 'categories' %>
-  <% @website.post_categories.ordered(current_language).each do |category| %>
+  <% block.website.post_categories.ordered(current_language).each do |category| %>
     <p><%= category.to_s_in(current_language) %></p>
-  <% end if @website%>
+  <% end if block.website %>
 <% else %>
   <% block.template.selected_posts.each do |post|
       l10n = post.localization_for(block.language)