refs
https://www.notion.so/ghost/Marketing-Milestone-email-campaigns-1d2c9dee3cfa4029863edb16092ad5c4?pvs=4
- Added email template for milestones with using a configuration file
for different member milestone values, as we're sending different
content for each one
- Implement sending the email to users who have
`milestone-notifications` enabled, currently still behind a flag
Co-authored-by: Peter Zimon <peter.zimon@gmail.com>
