Take a waiting list link in events' front matters into account

* An additional waiting list link can be given, which will be rendered
on the event's page and events' overview list.
* There are two new cases:
1) Event is booked out, there is no waiting list.
=> Display badge "fully booked out".
2) Event is booked out, but there is a waiting list.
=> Display waiting list button.
8 jobs for 140-display-url-to-second-event-as-waiting-list in 3 minutes and 39 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed build:latest #102846

00:02:25

passed build:production #102849

00:02:35

passed build:production:review-apps #102848

00:02:40

passed build:production:subpath #102847

00:02:31

 
  Test
passed test:htmlproofer #102850

00:00:58

passed test:htmlproofer:subpath #102851

00:01:00

 
  Review
passed review #102852
review-apps

00:00:06

passed stop_review #102853
review-apps manual

00:00:01