display up to 20 photos on inside Swipe container

'j/k' key bindings for 'next'/'prev' slide
Make things work on small screens

Switch from using width and height to max-width and max-height but set
`.photos` width to 100% so that it can render.
recalculate the slider in 'resize' window handler
There are some issues with margin still slider is rendered on a small
screen
Add prev and next buttons to the photo
cycle photos on next and prev

autoplay photos, until user starts clicking on them
rewind presentation when first/last photo is reached