Adding accessibility refresh to version 5.3

[audrasjb]

This was a very important project for the WordPress accessibility team and brought better accessibility on the whole admin interface. Also worth noting that several core team leads were against this change, so as the 5.3 accessibility release lead I had to defend that change for several weeks before it finally gets merged in Core. At the end, these changes are still here in WordPress admin, they provide better accessibility for everyone and WordPress has a better accessibility compliance now :)

For the record:

• Here is the dev note: https://make.wordpress.org/core/2019/10/18/noteworthy-admin-css-changes-in-wordpress-5-3/
• Here is the tests we had to put together to defend this change: https://make.wordpress.org/core/2019/10/18/noteworthy-admin-css-changes-in-wordpress-5-3/