Multisite: export/erase requests initiated in a sub-site should check whether the user belongs to that sub-site

[pbiron]

As of 5.4, when an export request is initiated from within a specific sub-site in a multisite, wp_user_personal_data_exporter() will export user data for a user even if that user is not registered on that sub-site.

wp_user_personal_data_exporter() should be modified to check that the user is registered on the sub-site where the request was initiated.

Note: wp_media_personal_data_exporter() and wp_comment_personal_data_exporter() do not need to be modified as the tables they query are specific to the sub-site, whereas the user table is global.

Related: #44176

[pbiron]

49668.diff limits user privacy exports initiated from a multisite blog (including the main site) to users registered on that blog.

A few notes:

1. the ! is_network_admin() check isn't needed as of right now, but I put it in there in anticipation of #43738, which is asking that export/erase requests be available at the network level
2. we'll have to think about how to make it easy for custom exporters to also limit exports in a similar fashion (which might just have to be in documentation...haven't given it much thought yet)
3. this is not yet ready to be committed, and should just be considered a POC at this point

[pputzer]

Replying to pbiron:

2. we'll have to think about how to make it easy for custom exporters to also limit exports in a similar fashion (which might just have to be in documentation...haven't given it much thought yet)

So this should probably be a utility function so that custom exporters don't have to reimplement the logic. Alternatively, the check should be moved ”up a level" to wp_privacy_process_personal_data_export_page().

[pbiron]

moving to 5.6, as not quite ready and we're already in 5.5 beta 2.

[garrett-eclipse]

5.6 beta is coming up quickly, will revisit in a future release.

[garrett-eclipse]

Dropping privacy focus as it's already in the Privacy component.