Accessibility: Remove title attributes from the Network Themes list table.

Title attributes in the "Themes" screen and in the "Edit Site" screen Themes tab
are now replaced with `aria-label` attributes. Also, replaces string
concatenation with `add_query_arg()` and `sprintf()` to allow translator
comments to be properly parsed and for better code readability.

Props SergeyBiryukov, afercia.
Fixes #35051.
Built from https://develop.svn.wordpress.org/trunk@35924


git-svn-id: https://core.svn.wordpress.org/trunk@35888 1a063a9b-81f0-0310-95a4-ce76da25c4cd