use cases
Social Login - Progressing Email
- Jon subscribes to newsletter with email jon@email.com , listed as UID-A
- Jon registers VIA twitter (without email) , listed as UID-B
- Jon updates his twitter account with email jon@email.com
- UID-A is merged into UID-B

Register - Using SiteUid
- Jon subscribes to newsletter with email jon@email.com , listed as UID-A
- Jon registers with email "jon@email.com" and siteUID "UID-B" , listed as UID-B
- UID-A is merged into UID-B

Import
- Jon subscribes to newsletter with email jon@email.com , listed as UID-A
- Jon imported from another system with email "jon@email.com" and uid "UID-B" , listed as UID-B
- UID-A is merged into UID-B

Notifications
