Audit and update "JavaScript work" epic

[domoscargin]

What

Review our JavaScript epic and ensure it's up to date, with all relevant tickets attached to it, or its child epics.

Related: https://docs.google.com/presentation/d/1FPaDB0brW_qTqD8TQQPjX6pQx1gDAm-2SAc4Vs52Mn8/edit#slide=id.gf5f38cbc77_0_113

Why

It's been a hot minute since we reviewed our JavaScript epic, and with new members joining the team it'd be good to get a grip on the overarching goals of the work and arrange the team's collective mind palace so we can prioritise work, plan a release schedule and break down the tasks.

Who needs to work on this

Developer

Who needs to review this

Tech Lead

Done when

• JavaScript epic is up to date (including work completed). It doesn't have to be 100% accurate to our upcoming plans.
• All related tasks added to the architectural changes board: https://github.com/orgs/alphagov/projects/41
• Issues and PR "sweep" to pick up any related work