Classic Editor: reduce height of slug editing field

[t-hamano]

Trac ticket: https://core.trac.wordpress.org/ticket/65063

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[t-hamano]

Note that this PR contains minimal changes for 7.0. Ideally, we should refactor using the flex layout. Appropriate spacing should also be added, for example, like this.

1200px width:

700px width:

360px width:

[sabernhardt]

Consider changing the OK button from button-small to button-compact. It would have the same height as the input on smaller screens when combined with the buttons.css change in #11549.

Then the Cancel button might have a larger font size than 11px, too (by removing that rule).