Allow a custom label to be specified for a slot. Show this label anywhere a slot is represented in a schedule view.

Benefit: It will be easier for event attendees in different time zones to be sure they're talking about the same slot.

https://discordapp.com/channels/692335950990278656/692335951506309153/706890155142807722
https://warhorn.groovehq.com/tickets/330
https://warhorn.groovehq.com/tickets/455

https://broadcast.warhorn.net/requests/en/slot-label-2MZ2xc4ti