add BROADCAST command and shortcut

behavior

no prefix (current default)

• when a user sends come to WSQ at 5pm today
  • then they get a command response saying Sorry, you didn't prefix your message with a valid command. To send out a message to all subscribers, use BROADCAST. To send a message only to fellow admins, use PRIVATE.

BROADCAST prefix / ! shortcut

• when a user sends out a message prefixed with BROADCAST come to WSQ today or !come to WSQ today`
  • then their message will be sent out to everyone

[BROADCAST]
come to WSQ at 5pm today

implementation

• new BROADCAST/! command
• change no-op behavior

note: change needs to be announced on BOOSTIT!