[89] Resolve "allow channel owners to toggle responses on/off"

Closes #89 (closed)

User-facing changes

GIVEN a channel on which responses are not enabled

• WHEN the owner sends a message that says "RESPONSES ON"
• THEN signalboost will enable subscriber responses on that channel

GIVEN a channel with responses enabled

• WHEN a subscriber sends a non-command message to the channel
• THEN it will be relayed to all channel owners

GIVEN a channel with responses enabled

• WHEN the channel owner sends a message that says "RESPONSES OFF"
• THEN signalboost will disable subscriber responses on that channel

GIVEN a channel with responses not enabled

• WHEN a subscriber sends a non-command message to the channel
• THEN they will receive a "Whoops!" message in response

GIVEN a channel

• WHEN a publisher send a message that says "RESPONSES FOO" (or anything other than ON/OFF)
• THEN they will get an error message notifying them of the allowed settings

GIVEN a channel

• WHEN a publisher issues an INFO command
• THEN they will see the current state of the subscriber responses toggle as part of the response

Implementation notes

• add a responsesEnabled field on channel
• check responsesEnabled in messenger dispatch flow (and relay message to owners if sender is a subscriber)
• add new TOGGLE_RESPONSES command to services.dispatcher.executor.js
• add new BROADCAST_RESPONSE message type to services.dispatcher.messenger.js
• generally refactor dispatcher.messenger for clarity and readability
• descoped command notificiations for all admins, but left TODO for how to accomplish it
• not totally happy with factoring of dispatcher.messenger.format ATM, but seems okay for now and unlikely to change too much so fine with some extra copmlexity there for a bit