Use flexible padding to keep login form visible on small screens

[macmanx]

Line 8478 of wp-admin.css lists the #login element as having a hard-coded 114px top padding value.

This is a bit of an annoyance for users with smaller screens (like mobile devices), as the login form requires scrolling, even though it could fit perfectly without padding (or with very little padding) on most smaller screens.

The login form should use flexible padding so that it fits all screen sizes with no scrolling required.

[iammattthomas]

In 26300:

Switch to a fluid padding-top on #login so it aligns nicely on small screens. Fixes #26130.