Contacts

[lightness]

WHAT

1. Introduce an ability to invite people in cost-x in general and to personal contact list.
2. "contact" relation should be symmetric: if A is contact of B, then B is contact of A

WHY

1. Building user-based contact list will be used in subsequent features, for example multi-user workspaces.

HOW

1. Endpoint to fetch list of contacts (available for all authenticated users)
2. Initially list is empty
3. Endpoint to search for another user by email (available for all authenticated users)
4. Endpoint to send to existing user invite in contact list. (available for all authenticated users)
5. Endpoint to accept/reject invitation to contact list. (available for target user - invite receiver)
6. Endpoint to remove contact from contact list (available for all authenticated users)
7. Endpoint to invite by email person who isn't yet user of cost-x
8. Endpoint to accept/reject invitation by email
9. If invite by email is accepted, new user should immediately be added to inviter contact list