ENH: Use videos that have the mid-slice as a first frame

Benefit: panels looks proper once the first frame is loaded
Cost: makes the slice lookup math messy
ENH: Make the description optional
Move color specs outside the JS code
ENH: Remove all layout specs that aren't critical

This allows for doing a 3-panel layout in a single row, by changing the
CSS only.
Simple script to convert a NIfTI volume into a movie

Just a draft needs work.

Requires Python, matplotlib and nibabel, as well as ffmpeg.
A bit more infrastructure.

Given all dependencies a simple 'make' in the root should yield demo
videos.