As a user isn't allowed to send a single emoji more than once.

As users aren't allowed to react with the same emoji more than once.

Add relation aggregation APIs

Add simple relations API

This will allow us to reuse the same structure when we paginate e.g.
relations

Allow client event serialization to be async

Expose DataStore._get_events as get_events_as_list

Use correct config option for ratelimiting in tests

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

This is in preparation for reaction work which requires it.

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

Fix CI after new release of isort