Ability to add event metadata

[ukutaht]

Changes

This PR adds ability to add metadata to custom events. Technically it prepares the database for the ability to also add metadata to pageviews since pageviews and events share the same schema (#320).

Interestingly, in one issue this feature is called 'metadata' and in another, it's called 'custom dimensions'. I've went with 'metadata' for now but I'm curious to hear everyone's thoughts on naming this feature.

Currently the tracker only exposes a way to add metadata to custom events through the following API:

plausible('Signup', {meta: {variant: 'A'}})

In the dashboard, this can be interacted with in two possible ways:

1. When you click on a goal (e.g. 'Signup') then in the conversions report you'll find a UI to break down the conversions according to your metadata keys. Something like 'Show me all signups broken down according to variant'

2. In the breakdown you can click on a metadata key-value pair to then filter the whole dashboard. So you can start combining filters like Complete Signup and Signup.variant is A

Tests

• Automated tests have been added

Changelog

• Entry has been added to changelog

Documentation

• Docs have been updated