Describe the bug
We just ran into an unexpected issue today, if you change the email address of a user, pages that had a Users field are no longer linked to the affected user.

I genuinely thought the Users field would save a list of persistent User IDs (i.e. the folder name that contains an individual user's files). If it had done this, renaming an email address wouldn't break existing pages (it breaks now because $field->toUsers() finds no matching email address.

What's Actually Saved in a Content File

Authors: 

- [email protected]

What I Would Expect to be Saved

Authors: 

- By4XbnMB

To Reproduce
Steps to reproduce the behavior:

1. Have a user with an email [email protected] ("Example User")
2. Go to a page with a Users field, select "Example User"
3. Change the email address for "Example User" to [email protected]
4. Attempt to do $field->toUsers() on the original page

Expected behavior
The user should still be accessible.

Kirby Version
3.1.x and 3.2.0