Use timestamps instead of booleans

[Sporiff]

Boolean values are often unnecessary if an action timestamp conveys the information. For example, we currently use a boolean isSubscribed field on subscriptions to convey whether the subscription is active. Instead, an unsubscribedAt timestamp would convey more information:

• If it's present, the user is unsubscribed
• If it's not present, the user is subscribed
• The updatedAt and unsubscribedAt fields can convey different information (the unsubscribedAt timestamp is a definite action taken at a specific time, the updatedAt value is updated whenever a record changes).