Add indexes to fields commonly used in queries to build emails

[elceebee]

Context

When I looked at the email queries earlier, it was clear could potentially send nudges to candidates who couldn't do anything with them (users whose accounts were blocked or locked for some reason). So we added those filters.

This PR just adds indexes to those fields to help speed up queries.

Changes proposed in this pull request

This PR just adds indexes to those fields to help speed up queries.

Guidance to review

Link to Trello card

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Required environment variables have been updated added to the Azure KeyVault