Admin bar: Update the Edit Site link

[carolinan]

Trac ticket: https://core.trac.wordpress.org/ticket/62368

This is the backport of WordPress/gutenberg#69271

This PR changes the Edit Site link in the admin bar to point to the top level (the "home") of the Site Editor, instead of opening the current template.

Based on feedback in https://core.trac.wordpress.org/ticket/62368, the Edit Site link also shows in the admin area.
Showing the link while in the admin area makes the admin bar more consistent and makes it easier to use, since you can use your muscle memory.

Testing Instructions
Activate a block theme.
Confirm that the Edit Site link shows in the admin bar, and that it leads to the Site Editor, without opening a specific template.
Activate a classic theme and confirm that the link does not show.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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