EES-5537 EES-5500 Notifier ContentDB connection and archived subscriptions fixes

[mmyoungman]

This PR does three things

• Allows the Notifier to connect to the Content DB
• When unsubscribing a user, we now also remove any subscriptions from associated archived subscriptions.
• Fixes the unsubscribe link for subscribers to archived publications

Details for why we decided to create the DB connection for Notifier are in the comments in EES-5500. The TLDR is that we consider it better to open a Content DB connection for Notifier than continue to pass everything Notifier needs in requests. And now we have a Content DB connection in Notifier, this opens up the possibility for other simplifications, as Notifier can now fetch the data it needs itself.

🚨 This PR requires faffy deploy steps detailed in EES-5537 🚨

edit: For this to all work, I had to manually create a "login" every database we use - so the local test data, dev, test, preprod and prod Content DBs:
CREATE LOGIN notifier WITH PASSWORD = 'ourbrandnewsecurepassword';