Allow calendars to filter events based on summary, description, etc

[kimadactyl]

User story

As a partner, I want to share only some events from a calendar in order that I can use the PlaceCal importer with minimum fuss.

As an organiser, I want to be able to make sure my site listings are as good as possible and aren't pulling through things which shouldn't be there.

This is mostly an issue where we are using a website feed and there's things like "private event" listed.

This could be fixed by allowing a filter on each calendar to exclude events meeting certain criteria.

Sample event

Acceptance criteria

• [ ] criteria one
• [ ] criteria two

Implementation notes & questions

Implementation plan

To be written by the developer

[kimadactyl]

nb: this is about filtering at the calendar level, not the frontend level