changes/<change-id>/assignee
GET returns an AccountInfo:
{
"_account_id": 1000000,
"email": "user@gerrit.com",
"username": "user"
}
PUT takes an account identifier: name-email, email, full name, "self" and
returns an AccountInfo for the user that is assigned after the call.
Change-Id: Icce1be90c6c9558950502fe24435317e8372bb64