This commit adds token_hash and scopes columns to email_tokens table.
token_hash is a replacement for the token column to avoid storing email
tokens in plaintext as it can pose a security risk. The new scope column
ensures that email tokens cannot be used to perform a different action
than the one intended.
To sum up, this commit:
* Adds token_hash and scope to email_tokens
* Reuses code that schedules critical_user_email
* Refactors EmailToken.confirm and EmailToken.atomic_confirm methods
* Periodically cleans old, unconfirmed or expired email tokens
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