Streamline x-talks navigation

[dontcallmedom]

Assuming some people will want to watch the talks serially, I think we should streamline a bit that workflow:

• right now, when hitting the next/previous arrows, one lands at the top of the page whose content doesn't vary from talk to talk - I think it would be best if one landed at least at the level of the title / talk navigation bar
• in "transcript" mode, it would be nice to show the prev / next navigation at the bottom of the transcript (where one lands once reading through)
• in "watch" mode, maybe find a way to nudge the watcher towards the next talk when the video ends?

[bert-github]

The next/previous buttons now go to the intro section (the title of the presentation) rather than the top of the page.

The buttons are now repeated below the presentation (though not just in transcript mode).

As for nudging: The video player doesn't seem to tell us when the video ends. I don't know if that is something that can be configured. The underlying player does send events, but they are currently not passed on to our page.

If we had such an event, it would be easy to display a link to the next talk.