Add subscribed/unsub_reason fields to waitlist

[leplatrem]

This will be necessary for an efficient implementation of #561 . Without that, we cannot synchronize unsubscriptions efficiently on Acoustic. Without a trace of records with subscribed=False, the only way to keep to waitlist subscriptions in sync in Acoustic is todelete all entries of the waitlist relational table for a given contact, and re-add their subscriptions.

With this PR, the waitlist implementation will follow what we want for newsletters in #562

• Add fields to model, and DB migration
• Set subscribed to false instead of deleting waitlist records
• Add dedicated tests with unsub_reason

[grahamalama]

Without a trace of records with subscribed=False, the only way to keep to waitlist subscriptions in sync in Acoustic is todelete all entries of the waitlist relational table for a given contact, and re-add their subscriptions.

Not necessarily. In #571 we discussed the idea of using some to_delete queue. That discussion was focused on deleting entire contacts, but a similar design might also be useful for removing people from waitlists.

If it's a requirement to capture unsub_reason though, (i.e. if marketing will use this column for some reason) then having this "soft delete" approach would make sense.