Add Matchmaker Notification

_This is a summary of the discusion in https://github.com/FAForever/server/issues/1024_

We want to have a notification in the client when someone that you could match with in the matchmaker is searching for a game.

The server should determine if someone in the player's matching range is searching and then send out a message to the client. The message should be a new message type.
It probably makes the most sense to create a new dedicated service to handle the checking and message sending. You could have a look at `BroadcastService` to get some inspiration.

Non-Coding tasks: Create a mockup (in photoshop or something similar) of how exactly the message would look like in the client and where it should show up.
This will guide the client implementation.