Provide anonymization/personal data removal helper functions

[allendav]

For example, come up with functions to provide replacement text for things like name and address fields that can be used by plugins when removing personal data. The ideal text will be recognizable by the administration as having been thus removed, e.g. "Anonymized User".

Since email addresses are at the core of our export and removal strategy, the anonymized email address function should also probably support a random hash or some such, e.g. anonymized-user-123456abcdef@… to avoid creating a situation where all anonymized content ends up pointing at a single fictitious user.

From the ticket scrub discussion in Making WordPress gdpr-compliance chat today.

[allendav]

Duplicate of #43545.