Change `email_opted_in_at` from `datetime_with_timezone` to `datetime`

What does this MR do?

Changes email_opted_in_at from datetime_with_timezone to datetime, to allow it to be null, and not be automatically set on update.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See issue #3150 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3150 (closed)