This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.

* Base the outline button :active color on the background, fixing a contrast issue

* Only apply focus outline to active when it's focused

Fixes #25127.

The download link for Entypo (on its website) is invalid. It's a Dropbox link which now results in a 403 error. The website was last updated in 2015 (as seen in the footer).

"Choose file ..." text is now written inside the label tag, so the translation is outside the css scope. 
$custom-file-text variable should only contain "Browse" translations.

Position: sticky is included in Edge 16

https://crbug.com/781344

Closes #23319. Both `.text-muted` and `.btn-outline-secondary` (and indeed all secondary items) make use of `$gray-600`. New value provides a contrast ratio of 4.69.

Closes #24341

Closes #25067