Update webinar landing page with link to recording

[chrifro]

Context The webinar event has passed and it shouldn't be possible to sign up for it anymore.

To do's

• [x] replace sign up form with a link to the YouTube recording
• [x] on click, the video should be shown in a pop-up at a larger size. We can continue using the YouTube embed style for that
• [x] change the time to "past event"

See mockup