[i18n] In computing page drift, consider parts from shortcodes and included pages?

[chalin]

Originally posted by @chalin in #6583 (comment):

Tangentially related question: Do we hash includes and track them for localization drift?

Yes, given that the include pages are under content, they get checked by our i18n check script to ensure that they get tagged, and any drift is reported by the script. (I don't have a mechanism to have the drift notice show up on the website when due to drifted includes. I'll have to think about that.)

Page parts might be included via shortcodes, as was done in #6583, or include statements. Not sure it's worth the effort, but if we can find a simple solution, that would be great.