setup-config.php should use RTL CSS files

[SergeyBiryukov]

Background: #29452

For RTL, we have install-rtl.css and buttons-rtl.css files. We use them in install.php, but not in setup-config.php. This leads to misplaced list, fields, and button (see the screenshots).

This is not a regression, but seems trivial to fix. We use wp_admin_css() in install.php, and it works fine in setup-config.php too.

[SergeyBiryukov]

buttons.css is already a dependency of install.css.

[nacin]

Ah yes, we can do this now that we're bootstrapping WordPress. No side effects noticed? It should be fine, as the only difference between setup-config.php and install.php now is just one has a DB connection and one doesn't.

[SergeyBiryukov]

Didn't see any side effects with 29464.patch​.

[nacin]

Works for me.

[SergeyBiryukov]

In 29670:

Use wp_admin_css() to enqueue stylesheets in setup-config.php.

fixes #29464.