[Meta] Improve the accessibility of the Gin theme

Thanks so much @rkoller for this epic accessibility review. Many of the issues here will improve usability of Drupal CMS as well as of course accessibility. Gin is more complex than many other themes. It is important here that we give users good defaults, while striving to keep the flexibility which is built into this great theme. I look forward to engaging to find ways we can incorporate these improvements into Drupal sites moving forward.

@rkoller thank you for putting this together 👏

It will take a while to get through them so I appreciate you added a meta as well 🤝