backfill/Email model

[ehfeng]

We need to backfill's Email with historical UserEmail.

Because the Email.email field is CIText, but UserEmail can easily have duplicates, not to mention case-insensitive duplicates, we wrap it in a try except.

[ghost]

	2 Warnings
⚠️	You should update CHANGES due to the size of this PR
⚠️	PR includes migrations

Migration Checklist

• new columns need to be nullable (unless table is new)
• migration with any new index needs to be done concurrently
• data migrations should not be done inside a transaction
• before merging, check to make sure there aren't conflicting migration ids

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