Accessibility improvements for admin breadcrumbs

[Thibaud Colas]

There are two nice and simple accessibility improvements we could make to the Django admin’s breadcrumbs:

1. (fixed in 872b61193b013a700ff88cf50f0eb2cf2c266ff7) Wrap them in a <nav aria-label="{% translate 'Breadcrumbs' %}>"> region, so they can be identified as a navigation landmark by screen reader users.
2. Structure the links as a <ol> list with <li> items for each crumb, so screen reader users know how many items there are (visually they don’t need any changes)
3. Switch the last item to also be a link, with a aria-current="page" attribute, so semantically the last item is marked as "current page" (again doesn’t necessarily need any visual change). Technically this would also work as a span but I believe switching to a link would be a nicer experience (consistent tabbing through all items in the breadcrumb)

All three improvements are based on the ​Breadcrumb ARIA authoring practices pattern. All can technically be done separately if desired.

[Florian Perucki]

PR: ​https://github.com/django/django/pull/16106

[Mariusz Felisiak <felisiak.mariusz@…>]

In 872b6119:

Refs #34041 -- Added navigation landmark to breadcrumbs in admin.

Thanks Thibaud Colas for pair programming.