Make autoplay a parameter of the video player

This will allow us to have multiple players on a page and choose which (if any) to autoplay.

I've also refactored the (now hideous) video player template, so that it takes a video player model (on the suggestion of @gklopper). It's much nicer now!

I've also removed duplicate logic for constructing the end slate url from the JavaScript - it's now required that you pass this in when you construct the player and becomes a data attribute, which I think will make it easier to understand in the future.
... ah yeah, I forgot the old behaviour of looking up in the page metadata whether it was a media page ... OK, easy enough to fix. Sorry!