Gutenberg 7.7.1 update on wpcom: fix back button URL

[simison]

With Gutenberg 7.7.1, this:

...is replaced with this:

We need to ensure getting back to the editor works equally well with W logo. We might just need fixing some selectors.

Be mindful of wpcom-FSE sites! You can get one via https://horizon.wordpress.com/start/test-fse