[UX] Admin bar: Consider replacing the home button with a "Back to site" button.

Describe your issue or idea

This is basically a request to have https://www.drupal.org/project/escape_admin in core instead of (or in addition to) the home button. From that project's description:

This is a Drupal 7 backport of the functionality added to the revamped toolbar: ~#787896: Add a link so that administrators can return to their most recently visited non-admin page~ which adds a 'Back to site' button when the user has clicked on an administration URL from a non-administration page. The 'Back to site' link will persist until the user returns to a non-administration page.

Steps to reproduce

The UX- with the current situation is that we are assuming that the user wants to return to the home page after dealing with any admin/backend task, whereas the most likely thing is that they want to return back to the page where they started from.

For example: as a novice admin, go to any page of your site -> realize that you want comments enabled -> edit the page -> realize that comments are not enabled for the page content type -> go to Structure -> Content types -> Page -> make the changes -> Save. Now, if you click the Home button, you are taken to the home page, while you most likely needed to navigate back to the page you started from.

Actual behavior

The "Home" button takes the user to their home page. This is fine, but they already have a "home" trail in the admin theme that they can use if they want to go to the home page.

Expected behavior

A much better UX would be to have it so that when the user navigates to any admin page from the frontend, the path of the page they started from is stored in their browsing session. This path persists as they navigate through the admin interface, until they hit the "Back to site" button. What happens then is that they are taken to the page where they started from. As the user navigates through the frontend, their session is updated to store the last non-admin page they visit before switching to the admin UI.

If we think that changing the current behavior of the "Home" button would be disrupting the way current users are used for things to work, then we can either make this an option in admin/config/administration/admin-bar or add a separate "Back to site" button.

---

PR by @alexfinnarn : https://github.com/backdrop/backdrop/pull/2040