Don't load admin files on each request

[dimadin]

In wp-settings.php, where WordPress is actually started, we load wp-includes/rest-api.php which loads wp-includes/rest-api/class-wp-rest-server.php which loads wp-admin/includes/admin.php which then loads a bunch of admin files and translation.

This is wrong for two reasons:

• We shouldn't load admin files on each request.
• It is against #33413.

I don't see why admin files are needed in WP_REST_Server though they might be needed by REST API. In that case, it should be lazy loaded somewhere.

[swissspidy]

This was introduced long time ago, back when @rmccue worked on it during GsoC. See ​https://gsoc.trac.wordpress.org/changeset/1980

Seems like it was related to some date handling functions. I ran the core tests and the plugin tests from GitHub now without including wp-admin/includes/admin.php and it worked without a problem. So either it really isn't needed or we just don't know the reason why it's still included.

[wonderboymusic]

In 35353:

REST API: don't load wp-admin/includes/admin.php on every request.

Props swissspidy.
Fixes #34395.