The automation plugin has 4 custom field types that are array typed. However, array typed custom fields are deprecated and should be migrated to JSON type.
This commit does a couple of things:
1. Migrate all four custom fields to JSON
2. Fix a couple of small bugs that have been discovered while migrating the custom fields to JSON (see the comments on this commit's PR for details https://github.com/discourse/discourse/pull/26939)
This commit adds a new option to the `user_updated` trigger of the automation plugin to only trigger an automation for new users that join after the automation is enabled.
Internal topic: t/125829/9.
