Fix follow activity insertion, impl undo follow

- Follow created a new id instead of using the Id of the activity for
  the db activity

storage-new/models/UserToUserRelation.go

@@ -9,5 +9,5 @@ type UserToUserRelation struct {
 	UserId       string
 	TargetUser   User // The user Y described in [RelationType]
 	TargetUserId string
-	Relation     string `gorm:"type:relation_type"`
+	Relation     string // `gorm:"type:relation_type"`
 }