Upcoming events
<% all_events.each do |event| %>
<% if (sub_events(event)).empty? %>
<%= render '/partials/_event_preview.*', event: event %>
<% else %>
<% sub_events(event).each do |sub_event| %>
<%= render '/partials/_event_preview.*', event: sub_event, main_event: event %>
<% end %>
<% end %>
<% end %>