MNTOR-1800: Part 1 - new table for subscriber email preferences

[mansaj]

References:

Jira: MNTOR-3583

Description

The work to migrate email preferences to a new table need to be done in a few steps:

1. create the table
2. handle migration of the data from old table (subscribers) to this new table.
3. Change the business logic to fetch from this new table / update new table

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions]

Preview URL 🚀 : https://blurts-server-pr-4972-mgjlpikfea-uk.a.run.app

[Vinnl]

I recall reading (but can't find) the advice to instead make this a nullable datetime column *_disabled_at, would that make sense here? It's not a table that's read or written to particularly often I don't think, so not too much of a load, but would e.g. allow us to notice weird unsubscription patterns, and/or correlate unsubscription spikes with certain events? (Alternatively, simple Glean metrics might be fine too.)

[github-actions]

Cleanup completed - database 'blurts-server-pr-4972' destroyed, cloud run service 'blurts-server-pr-4972' destroyed