Do not use hardcoded colors for calendar API

Job Doesburg
2020-03-04T17:06:42+01:00

### 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.