Don't send new users the monthly activity email

[Vinnl]

References:

Jira: MNTOR-3186
Figma: N/A

Description

The first email should only get sent after the user actually has activity, i.e. after the first month.

How to test

Create a new account, subscribe them to Plus (ensure they have the susbcription in the DB via /admin/dev). Make sure the MonthlyActivityEmail feature flag is enabled for them. Run the npm run dev:cronjobs script. It should not send the email to them - but when you switch to main, it does.

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. - N/A, DB query
• 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.

[rhelmer]

lgtm. I wish we had an easy way to unit test SQL changes.