Filter exhibitions

[gestchild]

What does this change?

For #331

Makes it possible to filter out events by their format type, e.g. format=!{id} or format=!{id-alias}

How to test

• visit https://api-dev.wellcomecollection.org/content/v0/events and note number of results
• visit https://api-dev.wellcomecollection.org/content/v0/events?filterOutExhibitions see fewer results
• visit https://api-dev.wellcomecollection.org/content/v0/events?format=!exhibitions see (the same amount) fewer results

How can we measure success?

We have a more flexible way of filtering by formats that follows the same pattern as the Catalogue API.

Have we considered potential risks?

Nothing really