Game mode support

[Qaomen]

Description
We have several special game modes planned that are currently listed as their own issues. But in order to properly support any of them, we first need other features that these game modes will utilize in turn.

Minimum Acceptance Criteria

• The server should manage a list of the various available game modes.
• The server should have logic to occasionally pick a special game mode at random.
• In the database, round data should include a game mode.
• Game modes should contain a flag whether it can affect personal bests and level world records.
• Game modes should be able to receive certain user commands (preferably, these commands are added automatically).
• The client should display game mode name and information/instructions during active special game modes.
• The client should differentiate between race entries and marble entries (e.g. for multi-marble and/or late-spawning marbles).
• Operators should be able to force game modes for the next round by using !gamemode [gamemode-name]

Is blocked by the following issues
Affected by #87 and #281