Privacy: Add an indication when the Copy action in Privacy Policy Guide is complete.

This adds a "Copied!" text near the "Copy this section to clipboard" button to provide direct feedback that the action was completed.

Props garrett-eclipse, nickylimjj, xkon, desrosj, birgire.
Fixes #44588.
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git-svn-id: https://core.svn.wordpress.org/trunk@47347 1a063a9b-81f0-0310-95a4-ce76da25c4cd