Accessibility: Plugins: Accessibility and CSS improvements for the Plugins pages.

- improves checkboxes alignment on the "Plugins" page table in the responsive view 
- improves spacing between form controls on the "Add Plugins" page in the responsive view 
- the layout of the "filter bar" on the "Add Plugins" page is now based on CSS Flexbox 
- removes italic type from a paragraph in the "Favorites" page

Props passoniate, garethgillman, maxpertici, audrasjb, sabernhardt, afercia.
See #47327.
Fixes #49231.

Built from https://develop.svn.wordpress.org/trunk@48281


git-svn-id: https://core.svn.wordpress.org/trunk@48050 1a063a9b-81f0-0310-95a4-ce76da25c4cd