Arena: Allow to disable ready/notready commands

[rtxa]

This could be useful if server owners feels like players are exploting this.

Possible issues:

1. Players can exploit this feature to win a match by scoring some points and then not playing for the rest of the game.
2. Players can get confused on why a player is not playing a match or being skipped.

Possible solutions:

1. Maybe this can be tackled by adding a add a cvar with the name of sv_ag_arena_disable_notready or sv_ag_arena_notready.
2. Maybe this can be fixed by adding a notification in the chat when a player has issued the command notready or ready. A cooldown would be nice to add too to avoid players flooding the chat. The cooldown time would be from 1 to 3 seconds.