Merge from staging #7

* This is a combination of 8 commits.
This is the 1st commit message:

> First WIP version of supporting nested subforms.

This is the commit message #2:

> Fix nested subforms not getting correct name attribute on input fields.

This is the commit message #3:

> Fix subform rows having invalid index, fix small typo.

This is the commit message #4:

> Replace the unique subform id via random bytes by just an increasing integer in the fields type rendering process.

This is the commit message #5:

> Implement feedback from PR at Joomla;
> - Use a <template> HTML element for the template of the subform rows,
>   not a url encoded string inside of a <script> element.
> - Fix code style errors reported by phpcs.
> - Make the fixing of the unique attributes (name, id, etc) of input elements
>   of nested subform rows more errorprone, using the same method as the main
>   subform row.
> - Manually add a minified version of the javascript file.

This is the commit message #6:

> Fix failing javascript tests due to changed structure of subform repeatable template container.

This is the commit message #7:

> Change subform repeatable javascript test to correctly check on
> 0-indexed rows, previously they have been 1-indexed.

This is the commit message #8:

> Fix a problem where multi-level subforms on the same level doesnt trigger their template correctly.
> Additionally added a note why the fixScripts() method is broken and how it could get better.

* Commit code styling errors and add an aria label for the outermost 'add' button.

* Fix double group-add button on repeatable subform type.

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