ServiceAccounts: Don't create new orgs for service accounts (#51819)

* Org: use constants for status codes

* ServiceAccounts: Avoid creating new orgs for service accounts

* Document createUserBehavior

* Update pkg/services/sqlstore/org_users_test.go

* add doc string to flag

pkg/models/org_user.go

@@ -102,6 +102,9 @@ type AddOrgUserCommand struct {
 
 	OrgId  int64 `json:"-"`
 	UserId int64 `json:"-"`
+
+	// internal use: avoid adding service accounts to orgs via user routes
+	AllowAddingServiceAccount bool `json:"-"`
 }
 
 type UpdateOrgUserCommand struct {