Upload: Fix Plugin and Theme upload form are misaligned.

This commit makes sure that when uploading a plugin or theme file, the Install Now button is correctly aligned, and the button does not drops to the next line if filename is long.

Props seedsca, SergeyBiryukov, audrasjb.
Fixes #48859.
Built from https://develop.svn.wordpress.org/trunk@47271


git-svn-id: https://core.svn.wordpress.org/trunk@47071 1a063a9b-81f0-0310-95a4-ce76da25c4cd