Create unique digests for email verification tokens

What does this MR do and why?

This fixes the ActiveRecord::RecordNotUnique errors that were thrown when saving email verification tokens.

Before this fix, when a random 6-digit code for email verification was generated, it was hashed and stored in the users.unlock_token or users.confirmation_token columns. Since those columns have unique indexes, an error is thrown when already existing values are inserted.

The key for hashing was always identical: a hash generated by ActiveSupport::KeyGenerator#generate_key, salted with the Devise.secret_key. The input was 'Devise unlock_token' or 'Devise confirmation_token', resulting in identical digests when 2 identical tokens for the same column are generated.

Given there are only 10^6 identical 6-digit tokens and given the Birthday problem, there exists a 50% chance of an identical token out of every 1178 generated tokens.

This fixes that issue by replacing the generic key with the user's email, thereby creating a unique digest for identical codes.

Issue: #408124 (closed)

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