Allow custom accordion open and close text

[HughePaul]

Try to read custom open and close button text from a data attribute and fall back to English text to enable localisation.

fix for #1325

[frankieroberto]

Nice! Feels a bit weird to be including html fragments within attributes (as you'd have to remember to escape any " marks) but maybe that’s ok.

The alternative might be to bake multi-language support (at least for Welsh?) into the javascript, and then use a data-lang (or just lang?) attribute to select the right language.

[hannalaakso]

Hi @HughePaul

Thanks a lot for your work on this 🙌 This is a totally valid approach which solves your particular use case, but we need to find a way to support localisation consistently in a way that will work for different types of components and for other languages. For that reason, we’re not going to be able to accept this pull request.

There's some related discussion here for context: #958 (comment).

We've captured your PR as part of a larger epic #1389 to improve how Javascript works in GOV.UK Frontend. For now, we suggest that you fork and vendor in this component and make the adjustments in the service you’re working on.

Thanks for taking the time to propose this, and for your patience.