Change select options from 6,9,12 to 3..30

The selector in highlighted assemblies only allowed to chose between 6,9
and 12, limiting the options of admins. This change allows for a more
flexible experience

decidim-assemblies/app/cells/decidim/assemblies/content_blocks/highlighted_assemblies_settings_form/show.erb

@@ -1,3 +1,3 @@
 <% form.fields_for :settings, form.object.settings do |settings_fields| %>
-  <%= settings_fields.select :max_results, [6, 9, 12], prompt: "", label: %>
+  <%= settings_fields.select :max_results, (3..30).step(3).to_a, prompt: "", label: %>
 <% end %>