Make events which have open signups use a different color in the maincalendar component

The maincalendar component is used at /calendar and /home, and it would be very nice to see at a glance which events are open for signup. You can use the same method for coloring the events as is used on the car booking pages and the members' cafe shift page. To check if an event is open for signup, you have to see if it's within the signup period as well as checking if it isn't closed (you can take inspiration from the SignupCard on the event details page if you want!).