Implement a few channel member endpoints for APIv4 (#5444)

* Implement POST /channels/members/{user_id}/view endpoint for APIv4

* Implement PUT /channels/{channel_id}/members/{user_id}/roles endpoint for APIv4

* Implement DELETE /channels/{channel_id}/members/{user_id} endpoint for APIv4

app/channel.go

@@ -700,7 +700,7 @@ func LeaveChannel(channelId string, userId string) *model.AppError {
 			return err
 		}
 
-		if err := RemoveUserFromChannel(userId, userId, channel); err != nil {
+		if err := removeUserFromChannel(userId, userId, channel); err != nil {
 			return err
 		}
 
@@ -765,7 +765,7 @@ func PostRemoveFromChannelMessage(removerUserId string, removedUser *model.User,
 	return nil
 }
 
-func RemoveUserFromChannel(userIdToRemove string, removerUserId string, channel *model.Channel) *model.AppError {
+func removeUserFromChannel(userIdToRemove string, removerUserId string, channel *model.Channel) *model.AppError {
 	if channel.DeleteAt > 0 {
 		err := model.NewLocAppError("RemoveUserFromChannel", "api.channel.remove_user_from_channel.deleted.app_error", nil, "")
 		err.StatusCode = http.StatusBadRequest
@@ -797,6 +797,22 @@ func RemoveUserFromChannel(userIdToRemove string, removerUserId string, channel
 	return nil
 }
 
+func RemoveUserFromChannel(userIdToRemove string, removerUserId string, channel *model.Channel) *model.AppError {
+	var err *model.AppError
+	if err = removeUserFromChannel(userIdToRemove, removerUserId, channel); err != nil {
+		return err
+	}
+
+	var user *model.User
+	if user, err = GetUser(userIdToRemove); err != nil {
+		return err
+	}
+
+	go PostRemoveFromChannelMessage(removerUserId, user, channel)
+
+	return nil
+}
+
 func GetNumberOfChannelsOnTeam(teamId string) (int, *model.AppError) {
 	// Get total number of channels on current team
 	if result := <-Srv.Store.Channel().GetTeamChannels(teamId); result.Err != nil {
@@ -847,7 +863,7 @@ func SearchChannelsUserNotIn(teamId string, userId string, term string) (*model.
 	}
 }
 
-func ViewChannel(view *model.ChannelView, teamId string, userId string, clearPushNotifications bool) *model.AppError {
+func ViewChannel(view *model.ChannelView, userId string, clearPushNotifications bool) *model.AppError {
 	if err := SetActiveChannel(userId, view.ChannelId); err != nil {
 		return err
 	}