refs https://github.com/TryGhost/Team/issues/1808
refs https://github.com/TryGhost/Team/issues/1809
refs https://github.com/TryGhost/Team/issues/1820
refs https://github.com/TryGhost/Team/issues/1814
### Changes in `member-events` package
- Added MemberCreatedEvent (event, not model)
- Added SubscriptionCreatedEvent (event, not model)
### Added `member-attribution` package (new)
- Added the AttributionBuilder class which is able to convert a url history to an attribution object (exposed as getAttribution on the service itself, which handles the dependencies)
```
[{
"path": "/",
"time": 123
}]
```
to
```
{
"url": "/",
"id": null,
"type": "url"
}
```
- event handler listens for MemberCreatedEvent and SubscriptionCreatedEvent and creates the corresponding models in the database.
### Changes in `members-api` package
- Added urlHistory to `sendMagicLink` endpoint body + convert the urlHistory to an attribution object that is stored in the tokenData of the magic link (sent by Portal in this PR: https://github.com/TryGhost/Portal/pull/256).
- Added urlHistory to `createCheckoutSession` endpoint + convert the urlHistory to attribution keys that are saved in the Stripe Session metadata (sent by Portal in this PR: https://github.com/TryGhost/Portal/pull/256).
- Added attribution data property to member repository's create method (when a member is created)
- Dispatch MemberCreatedEvent with attribution
### Changes in `members-stripe-service` package (`ghost/stripe`)
- Dispatch SubscriptionCreatedEvent in WebhookController on subscription checkout (with attribution from session metadata)
