Improved options for video elements, and a `{video}` directive

[rowanc1]

Right now it is possible to add videos through an .mp4, and the default is to loop, mute and hide the controls.

We should open up the options to allow for turning the controls on, not auto-playing, etc. Doing this means a distinct directive and spec component. Currently we are seeing video as an image node with .mp4, and that falls down in a lot of places.

I think there are five things to take videos over the line: (1) new directive; (2) new AST node; (3) rendering that in the theme; (4) having upgrade logic for the spec; (5) testing the places where we fallback to static images (e.g. PDF).

cc @gvarnavi @cophus @fperez