Add /blitzkrieg game mode

[FRQDO]

This command tries to start a game in Blitzkrieg mode, optionally with a specified timeout in minutes. The default value could be set to 5, but maybe also be set on a per-channel basis.

When starting in Blitzkrieg mode, every player in the current lobby receives a message and button to confirm that they’re up for a Blitzkrieg match, and no new players may join. If not all players confirm within the timeout period, the game returns to the classic Lobby mode.

If a game runs in Blitzkrieg mode, every game interaction has a timeout. If the time runs out, the game is canceled, everyone loses, and the players who failed to perform their actions in time get one strike.

Players with three strikes are banned from Blitzkrieg matches.
If such a player is in the lobby while the /blitzkrieg command is issued, the bot tries to start the game with just the un-banned players.

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Example 1:
6 (un-banned) players in the lobby
→ Blitzkrieg match can start with 6 players

Example 2:
8 players in the lobby, of which 1 is banned
→ Blitzkrieg match can start with 7 players

Example 3:
7 players in the lobby, of which 3 are banned
→ Blitzkrieg match cannot start because there are less than 5 un-banned players in the lobby.

[FRQDO]

Open questions:

• How do people lose their strikes again?
• What is the minimum / maximum timeout?
• What happens if players try to /leave or /join during the Blitzkrieg confirmation phase?

[Eisfunke]

• Maybe a strike could expire after, let's say, 1 month without getting any new strikes.
• 5 to 30 minutes per action, 10 default?
• Blitzkrieg mode is resetted and has to be started again?