Link/embed Youtube videos when available

[erwin1]

• The youtube URL will be a property of the talk in the API
• When it becomes available, we can show a link to (or even better, embed) the video
• Additionally, we can show a (local) notification if:
  ** the user had favorited the talk
  ** but didn't vote for it (we then assume he was interested, but didn't make it in the talk)