PLT-3462 Add the ability to clear push notifications after channel is viewed (#3834)

* Add the ability to clear push notifications after channel is viewed

* Fix race condition between updating the mention count and reading it when sending push notifications

store/sql_user_store.go

@@ -921,3 +921,22 @@ func (us SqlUserStore) GetUnreadCount(userId string) StoreChannel {
 
 	return storeChannel
 }
+
+func (us SqlUserStore) GetUnreadCountForChannel(userId string, channelId string) StoreChannel {
+	storeChannel := make(StoreChannel)
+
+	go func() {
+		result := StoreResult{}
+
+		if count, err := us.GetReplica().SelectInt("SELECT SUM(CASE WHEN c.Type = 'D' THEN (c.TotalMsgCount - cm.MsgCount) ELSE cm.MentionCount END) FROM Channels c INNER JOIN ChannelMembers cm ON c.Id = :ChannelId AND cm.ChannelId = :ChannelId AND cm.UserId = :UserId", map[string]interface{}{"ChannelId": channelId, "UserId": userId}); err != nil {
+			result.Err = model.NewLocAppError("SqlUserStore.GetMentionCountForChannel", "store.sql_user.get_unread_count_for_channel.app_error", nil, err.Error())
+		} else {
+			result.Data = count
+		}
+
+		storeChannel <- result
+		close(storeChannel)
+	}()
+
+	return storeChannel
+}