Do not use hardcoded colors for calendar API

One-sentence description

Do not use hardcoded colors for calendar API in website/members/api/serializers.py r42-49 and website/events/api/serializers.py

Why?

We shouldn't hardcode these things at the API level but provide them using CSS.

Current implementation

The API provides a backgroundColor and textColor in the event sources APIs that is used by FullCalendar via https://fullcalendar.io/docs/events-json-feed. Every item in the feed is an event source object: https://fullcalendar.io/docs/event-source-object.

Suggested implementation

Provide a className attribute instead and determine the background color and text color based on the class using CSS.