Add key for schedule icons

[SamLR]

The icons on the schedule (i.e. CW, family friendly, not recorded) all have alt-text but this isn't always visible (e.g. on mobile). Add a key somewhere

[dave1010]

Looks like this was done in 2bb8ddc

https://github.com/emfcamp/Website/blob/main/templates/schedule/_icon_key.html

[marksteward]

This was only done on the favourites/line-up page. It needs adding to the main schedule too.