Nunjucks templates from GOV.UK Frontend plugin used internally

[colinrotherham]

Description of the issue

Manage prototype pages use GOV.UK Frontend "internal" styles, scripts, images, fonts etc

But they're rendered using GOV.UK Frontend "plugin" Nunjucks views and macros

Steps to reproduce the issue

1. Create a new prototype which uses [email protected] by default
2. Downgrade "plugin" GOV.UK Frontend using npm install [email protected]
3. Start the prototype with USE_HTTPS=false PASSWORD=test to see the password page
4. Click "Continue" to see the Error summary component
5. Notice that fixed accessibility bugs have returned

Actual vs expected behaviour

Actual: Nunjucks templates from "plugin" GOV.UK Frontend are used
Expected: Nunjucks templates from "internal" GOV.UK Frontend are used

[colinrotherham]

This was flagged whilst testing GOV.UK Frontend v5

Noticeably the class .js-enabled was missing its new .govuk-frontend-supported counterpart

[colinrotherham]

Currently in review: #2355