Add visibility conditions for the "login pages" and admin paths (and negation for them as well)

Description of the need

It would be nice to have a visibility condition that would allow us to determine when we are one one of the three login pages: user/login, user/register, and user/password.

This would enable us to use a special layout only on these pages, or add blocks to another layout that should either be only shown, or only hidden on these pages (especially useful when the blocks are intended for logged-in-eyes only).

If https://github.com/backdrop/backdrop-issues/issues/4410 gets merged, I would also like to recommend that add this condition automatically to any block placed onto the default administrative layout, in an effort to help keep people safe. It would be easy to accidentally leak private information to the public if you did not realize the default admin layout would be used on login-pages when you enabled that setting.

Proposed solution

• Create another visibility condition for Layouts
• Assign the visibility condition automatically to any block placed onto the default administrative layout