demo-stb: Download assets as part of docker build

Takes advantage of docker's caching so the large video files only need to
be downloaded once.
fixup! demo-stb: Download assets as part of docker build
fixup! demo-stb: Add SVG remote control for web-ui
demo-stb: Add missing key to key-mapping
demo-stb: Remove help icons

This is to be used in the stb-tester web-ui, which has a picture of a
remote control with clickable buttons.
demo-stb: The "home" button closes the home screen

This is to avoid having an extra "close" button on the remote control in
the web-ui.