Implement "subscription expiring soon" messages

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┆Issue is synchronized with this Jira Task ┆author: Lesley Norton

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➤ Santiago Andrigo commented:

Update: In today’s UX meeting we decided to deprioritize this for 2.10 for the following reasons:

1. This message is not required to reach feature-parity with the previous (pre AddOn infrastructure) messages experience. In other words, releasing without this feature is not worse off.
2. This message is not trivial to implement because the information about the subscription end-date is not always available. A hack would be required if we wanted to slot this in for 2.10. Florian knows the details of this.
3. The function of announcing to the user that their subscription is expiring is already (at least partially) being satisfied by the email we send when this situation happens. Moreover, at this point we don’t know how much more incremental value we’d get since we have no baseline metrics to understand how much users engage / read the messages we send.

We can revisit this once we launch the messages and get a better understanding about it’s effectiveness as a means of communication to users.