Use get_user_locale() in Customizer

[swissspidy]

In wp-admin/customize.php, get_locale() is being used to add a locale-$sitelocalebody class, which leads to some CSS being applied to the customizer even though the locale isn't used.

To reproduce:

1. Set user language to en_US
2. Set site language to anything else, e.g. de_DE
3. Notice that the body class is locale-de-de and therefore some specific CSS for de_DE is being applied, even though all the text in the customizer is in en_US.

[swissspidy]

In 40368:

Customize: User get_user_locale() in customizer body class.

Otherwise CSS specific to the site's locale would be applied, even though the customizer is displayed in the user's locale.

See #29783.
Fixes #40271.

[swissspidy]

In 40369:

Customize: Use get_user_locale() in customizer body class.

Otherwise CSS specific to the site's locale would be applied, even though the customizer is displayed in the user's locale.

See #29783.
Fixes #40271.

Merges [40368] to the 4.7 branch.