Investigate full text search performance

[Rikuoja]

Check whether https://docs.djangoproject.com/en/3.0/ref/contrib/postgres/search/#performance full text search would be more performant in straightforward text contains queries https://github.com/City-of-Helsinki/linkedevents/blob/master/events/api.py#L679

The full text search would need to be significantly faster than the icontains query for a production database with approx. 150 000 events, of which some thousands are in the future.

Therefore, the expected query is ?text=some_text&start=today, which filters events to those taking place in the future. If the full text search is not easily combined and fast with other filters narrowing down the queryset first, we have no use for it.