Emergency shutdown: instead of actual termination, continue working but ignore any commands

[kolayne]

Right now the emergency shutdown makes the Rimokon process actually quit. This introduces inconvenient side effects that should be resolved by running after_shutdown.py. The idea is to reimplement emergency shutdown: instead of quitting, the bot will continue to receive and process messages, however, it won't perform any commands with them (or just reply that the bot is shut down and should be restarted manually).