Create new command system

[ikkentim]

Requirements:

• (most) code should be shared between GM/ECS
• should support RCON commands and player commands
• command groups
• command overloads
• permission requirements for player commands
• should be able to enumerate commands
• should provide easy way to override "usage" message
• should provide easy way to write help commands
• should be backwards compatible with the old GM player command system - or - should work alongside the old GM player command system (in this case we'd deprecate and remove the old GM player command system in a future version)
• should provide full support for asynchronous task handlers (including Task/Task<T> return types)
• Localization support for usage messages
• [GM] Allow commands to be defined in a controller
• Should be fully covered by unittests

This should address #203, #269, #329 and #392.

Will close #393, #356