[WP][UX] Use admin-theme for Log In form

Description of the need

There should be a setting to use the admin theme for the log-in page, similar to how we have a checkbox to use the admin theme for the content edit form.

This would prevent everyone from needing to theme the log in page specially, when the log in form is only used for administrators.

Proposed solution

• Add a checkbox
• Switch the log-in / password reset / registration pages to use the selected theme
• Make sure these pages look good in the Seven theme

Related issues

• https://github.com/backdrop/backdrop-issues/issues/1528 Allow a layout to be selected on multiple paths
• https://github.com/backdrop/backdrop-issues/issues/4476 Add a "login pages" visibility condition

Blocking issue * https://github.com/backdrop/backdrop-issues/issues/4487 Remove the 'view administration theme' permission

Advocate

Jen Lampton

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PR: https://github.com/backdrop/backdrop/pull/3147