Theme editor nonce should be based on the relative file path

[dd32]

Currently the theme editor nonce is based on the full file path of the files, for example:

edit-theme_/www/wordpress/src/wp-content/themes/twentyseventeen/style.csstwentyseventeen

It'll make much more sense when debugging code if it's based on the relative path, which is how we validate it in the rest of the code:

edit-theme_twentyseventeen_style.css
edit-theme_twentyseventeen_inc/back-compat.php

Noticed from #42609

[dd32]

In 42246:

Theme Editior: Base the nonce on a simpler combination of fields, for easier debugging & reading.

See #42609.
Fixes #42705.