Check if next_event is present

[Plsr]

If there are no upcoming events (like today, last Webmontag was yesterday, no upcoming events are registered), there are some errors with the next_event object.

This affects two files:

• shared/content/_event_box.slim:

.event-box-inner
  h3
    =I18n.t('events.next.title', locale: locale)
  .event-date
      a href="events/#{next_event.date}"

.event-attend-buttons
    a.button.default href=next_event.facebook_url
      |Über eventbrite anmelden

• shared/index/_intro.slim:

h2
    = next_event.name

For no, I just checked for presence on every occurrence, maybe we should do that somewhat prettier.