[558] Add email to candidates about login changes DO NOT MERGE

[elceebee]

Context

We wish to email candidates in the imminent week(s) that the way they login to our service is changing.

Recognising that lots of new candidates sign up each day, we will send this email daily to new sign ups until the feature flag is activated.

Once the feature flag is activated and things are working normally, we will delete this email and worker from the code base as it's not to be reused. Even if we deactivate the feature flag, it won't really make much sense...

The first email that is sent will go to about 53k candidates. Subsequent messages sent daily will just be sent to candidates who have signed that day.

The message will not be sent to anyone whose account is locked, submission blocked, or unsubscribed.

Changes proposed in this pull request

Worker and email and associated tests.

Guidance to review

• Does the query look correct?
• The email can be previewed locally or on the review app at rails/mailers/candidate_mailer/one_login_is_coming

Link to Trello card

https://trello.com/c/ZAtlIyJO

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, if included inform data insights team of the changes
• If this code adds a column that may include PII, the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated.
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card

[CatalinVoineag]

Just a tiny comment. Looks good