allow channel owners to toggle responses on/off

Value

As an organizer, I want people to be able to respond to my messages without flooding the group so that I can gather valuable organizing information without having to give out my phone number or add people to a whatsapp group.

Acceptance Criteria

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
• AND THEN all owners will receive a message that says "Responses enabled on the channel by "

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
• AND THEN all owners will receive a message that says "Responses disabled on the channel by "

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

Implementation notes

• add a responsesEnabled field on channel
• check responsesEnabled in messenger dispatch flow (and relay message to owners if sender is a subscriber -- isntead of always responding with error message)
• add multi-owner notifications (currently only the owner who sent a command gets notified about its result)

Out of Scope

• if pressed for time, perhaps descope the multi-owner notification bit