Admin nav menu "Open link in a new window/tab" lacks spacing on top

[tyxla]

In admin nav menus, the expanded item "Open link in a new window/tab" field lacks spacing on top, which causes it to be positioned too close to the top fields.

After further investigation, I discovered that this comes from incorrect height that is set to .description-thin. If this height is removed, the .description-thin will grow to as much height as necessary, which fixes the issue.

[tyxla]

Removing hardcoded height from .description-thin, allowing it to grow as much as necessary.

[SergeyBiryukov]

Like with #31425, it's always a good idea to add screenshots for UI changes :)

Looks good, see 31426.png​.

[SergeyBiryukov]

In 31524:

Menus: Remove fixed height from .description-thin fields.

props tyxla.
fixes #31426.