Administration: Resize classic editor slug field for new theme.

Reduces the size and improves the alignment of the post slug field following the re-design of form elements as part of the new admin theme.

Props wildworks, sabernhardt, audrasjb, dhruvang21, shailu25, joedolson, khushdoms, tusharaddweb.
Fixes #65063.



git-svn-id: https://develop.svn.wordpress.org/trunk@62263 602fd350-edb4-49c9-b593-d223f7449a82