Make email deduplication more robust

[joebon]

Email deduplication (used to avoid sending the same email multiple times when for instance sending reminder emails) is currently brittle and unoptimized:

• Uses non-indexed metadata on email_logs table to figure out if email has already been sent

Should reasonably be fixed in a 3-step rollout:

1. Add a nullable deduplication key column to email_logs, along with code that populates it for new emails and with a backfill script. Nullable should be kept because not all emails require a deduplication key
2. Add a partial unique index
3. Switch from the current checking to checking the new key