#64832 Fix Upload Plugin form layout use flex box for file and submit

[PANawkar]

Fix the Upload Plugin/theme upload layout alignment

Wrap file input and Install Now in wp-upload-form-box; style as flex row so the button stays in the same grey box (fixes fr_FR and
avoids absolute positioning with top: auto, dropping the submit below). Update themes.css and themes-rtl.css; stack on narrow viewports.

Trac ticket: https://core.trac.wordpress.org/ticket/64832

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[joedolson]

This change just restores the problem previously fixed in https://core.trac.wordpress.org/ticket/64065. Let's try to find a solution that allows us to keep the larger drop area.