custom background images

We have a nice set of background images deployed for fairmeeting.

We could do the same for fairteaching, see howto.

We would need to adapt the ansible role.

Current background images:

https://fairteaching.net/html5client/resources/images/virtual-backgrounds/coffeeshop.jpg
https://fairteaching.net/html5client/resources/images/virtual-backgrounds/home.jpg
https://fairteaching.net/html5client/resources/images/virtual-backgrounds/brickwall.jpg
https://fairteaching.net/html5client/resources/images/virtual-backgrounds/board.jpg
https://fairteaching.net/html5client/resources/images/virtual-backgrounds/architecture.jpg