Use a variable for home URL on Permalink Settings screen

[sabernhardt]

As noted on #56110, I think replacing several instances of get_option( 'home' ) with a variable could improve the code.

It could also use the home_url() function, with escaping.

$url_base = esc_url( home_url() );

[SergeyBiryukov]

I ended up refactoring the whole file in [53710] / #55647 and only saw this ticket after committing. Sorry for that!

We could still use this ticket to replace get_option( 'home' ) with home_url().

[SergeyBiryukov]

It looks like esc_url() might not be needed here, as the value is not used in an href or src tag. However, home_url() does appear to be a more common way of retrieving the home URL across the admin.

[SergeyBiryukov]

In 53712:

Permalinks: Use home_url() to set the URL base on Permalink Settings screen.

This brings more consistency with how the home URL is retrieved elsewhere in the admin.

Follow-up to [1567], [1664], [2746], [3029], [4144], [53710].

Props sabernhardt.
Fixes #56235.