refs
https://www.notion.so/ghost/Marketing-Milestone-email-campaigns-1d2c9dee3cfa4029863edb16092ad5c4?pvs=4
- Added a `slack-notifications` repository which handles sending Slack
messages to a URL as defined in our Ghost(Pro) config (also includes a
global switch to disable the feature if needed) and listens to
`MilestoneCreatedEvents`.
- Added a `slack-notification` service which listens to the events on
boot.
- In order to have access to further information such as the reason why
a Milestone email hasn't been sent, or the current ARR or Member value
as comparison to the achieved milestone, I added a `meta` object to the
`MilestoneCreatedEvent` which then gets accessible by the event
subscriber. This avoid doing further requests to the DB as we need to
have this information in relation to the event occurred.
---------
Co-authored-by: Fabien "egg" O'Carroll <fabien@allou.is>
