Allow variants for action title, text, image, etc. and setup A/B testing

[mfb]

At some point we'd like to allow variants for action title, summary, description, image etc. and setup A/B (or possibly multivariate) testing. The variants could be applied and tracked via javascript (allowing action pages to be cached) - or if action pages are not cached, then the pages themselves could vary.

Tracking could be done by sending events to Matomo, e.g. using the content tracking API: https://matomo.org/docs/content-tracking/ or using the premium-only A/B testing API: https://matomo.org/docs/ab-testing/