Wishlists for v5.3+ and v6

[motin]

I'll start thread this by adding mine:

• Properly supporting cleanup on study endings (ie all the improved support for better endings -> utils) including:
  • Protection against normandy and user hard uninstalls which do not leave room for cleanup
  • hidden: true by default for all shield studies (in the template is fine)
• Be able to use Normandy devtools to install/revoke studies, test upgrades, expiration etc
• Signing using dev certificate during CI (for QA-ready xpi:s)
• Performance regression testing for add-ons
• A listing of all previous (29 in total?) shield study add-ons + those being engineered currently
• Add tests and test utils for testing persistent and restart features
• Clean and up to date documentation already on release day
• Typescript definitions