Update color indication in speaker availability selection when toggling checkboxes

[flummer]

Change the color indications in the browser when the checkboxes are checked (into green) or unchecked (into red). Either with pure CSS or maybe with javascript if absolutely needed.

It could maybe be the border of the checkbox or an attached label instead of the background of the parent element, to make it work with pure CSS.

In the case of expanding the availability submission (eg. adding evening/night times on top of already submitted daytimes) we need to find a suitable way to indicate to the user that the unchecked boxes in the new time blocks (normally marked with yellow) will be considered "unavailable" (red) and not "missing info" (yellow).