Includes:
- in gulpfile.js:
- add details macro to list of those copied from GOVUK Frontend
- remove existing details polyfill
- convert all, but one, <details> tags to use GOVUK Frontend details component
- add jinja boolean filter to help setting 'open' attribute of <details> tags
Notes on the `<details>` not included in this:
The `<details>` used for notifications items on
the API integration page is not possible to
reproduce with the GOV.UK Frontend macro so I'm
splitting the resulting work out into it's own
commit.