Feature/#607 match confirmation

[Askaholic]

Adds the ability for players to accept matchmaker games before the server tries to launch them. Clients will need to respond with a match_ready command which has already been supported by the server with a dummy implementation for years. If some players don't accept the match within the timeout, the match will be cancelled, the players who failed to accept the match will receive a matchmaker violation and their parties will be removed from all queues. Any parties where all players accepted the match will be automatically re-entered into the queues they had been searching for before the match was found.

New server messages:

• match_info: sent to create or update the match offer UI. This might also include additional information about the match in the future.
  Example:

  {
      "command": "match_info",
      "expires_at": "2020-01-31T14:30:36+00:00",
      "expires_in": 20.0,
      "players_total": 6,
      "players_ready": 2,
      "ready": false
  }

• match_notice: sent to players who are searching together to notify them which other player(s) in their party did not accept the match.
  Example:

  {
      "command": "match_notice",
      "unready_players": [1697983, 149808, 1390]
  }

Required client messages:

• match_ready: response to match_info with attribute ready=False to mark ones self as ready. All players in the match must respond with a match_ready message before the game launch sequence will begin.
  Example:

  {
      "command": "match_ready"
  }

Requires client changes as described in FAForever/downlords-faf-client#1783.
Closes #607

[Askaholic]

Can we just reuse the same object? That way we preserve the failed matching attempts / threshold expansion which would give the auto requeue a real technical benefit over just having people rejoin the queue.

[Askaholic]

Seems like yes