Use upstream slack-go/slack again (#1018)

vendor/github.com/slack-go/slack/websocket_subteam.go

@@ -0,0 +1,35 @@
+package slack
+
+// SubteamCreatedEvent represents the Subteam created event
+type SubteamCreatedEvent struct {
+	Type    string    `json:"type"`
+	Subteam UserGroup `json:"subteam"`
+}
+
+// SubteamCreatedEvent represents the membership of an existing User Group has changed event
+type SubteamMembersChangedEvent struct {
+	Type               string   `json:"type"`
+	SubteamID          string   `json:"subteam_id"`
+	TeamID             string   `json:"team_id"`
+	DatePreviousUpdate JSONTime `json:"date_previous_update"`
+	DateUpdate         JSONTime `json:"date_update"`
+	AddedUsers         []string `json:"added_users"`
+	AddedUsersCount    string   `json:"added_users_count"`
+	RemovedUsers       []string `json:"removed_users"`
+	RemovedUsersCount  string   `json:"removed_users_count"`
+}
+
+// SubteamSelfAddedEvent represents an event of you have been added to a User Group
+type SubteamSelfAddedEvent struct {
+	Type      string `json:"type"`
+	SubteamID string `json:"subteam_id"`
+}
+
+// SubteamSelfRemovedEvent represents an event of you have been removed from a User Group
+type SubteamSelfRemovedEvent SubteamSelfAddedEvent
+
+// SubteamUpdatedEvent represents an event of an existing User Group has been updated or its members changed
+type SubteamUpdatedEvent struct {
+	Type    string    `json:"type"`
+	Subteam UserGroup `json:"subteam"`
+}