Privacy: Give progress indication for export and erasure.

This adds a progress indicator for "Download Personal Data" and "Erase Personal Data" row actions, which can take a while with a lot of data.

Props garrett-eclipse, allendav, dominic_ks, xkon, karmatosed, birgire.
Fixes #44264.
Built from https://develop.svn.wordpress.org/trunk@47246


git-svn-id: https://core.svn.wordpress.org/trunk@47046 1a063a9b-81f0-0310-95a4-ce76da25c4cd