feat: use kartoffel users

[ShragaUser]

We should migrate to use a central user base ( kartoffel ) instead of our local user base ( users collection ).
This will allow us to "search" users who haven't signed up yet and integrate better with external services ( KDrive + Friends etc' )

[ShragaUser]

1. create migration script to modify user collection to use kartoffel mongo id as primary key
2. merge existing dual users ( for kartoffel users with multiple domain users )
3. change user id references in boards collection
4. figure out HI.Chat integration ( can they use kartoffel ids? )

[ShragaUser]

this is a pre-existing requirement for:
#29
#33
#43