[Notif] Invitations in RSS feed

[dakky21]

Concise description:

• Add message when you are invited to a game into RSS feed

Specifics/Details:

• Now RSS feed for a user lists only active games, why not use it also for invited games

How this will benefit the site and/or other comments:

• This way user can quickly check if he is invited to a game, rather than opening the site itself or checking e-mail