Add video preview image for Media Finder form widget

[bennothommo]

This PR adds a video preview image for mediafinder fields that have their mode config set to video. It renders the video screenshot by loading the first 10 seconds of the video and uses a HTML5 canvas element to take a snapshot as an image (stopping the video load afterwards).

In the course of this, I have also rewritten Media Finder to a Snowboard widget.

Only things left to do are:

• Make sure it works in preview and update contexts
• Perhaps cache or save the preview image so it doesn't have to load the video.
• Put something in place if the video cannot load (placeholder image perhaps?)
• Add preview for audio mode.

[LukeTowers]

Something that I'd like to do as a general pattern across all our frontend development is to ensure that all assets live under either dist/ or src/ folders, with the objective being that /src folders aren't expected to exist on production servers or be persisted to CDNs.

So assets/dist/js vs assets/js/dist

[LukeTowers]

What will the preview be for an audio file? Was this missed here?

[bennothommo]

Ah, I do intend to add an audio player to the backend field preview if an audio file is selected.

[LukeTowers]

With this and #958 it's looking more and more like we need to get #16 done as the thumbnail is definitely something that should stored in the item's metadata.

[LukeTowers]

@bennothommo what's the status of this?

[bennothommo]

I've held this work until we have media library metadata in place.