limit metadata in admin notifications and INFO responses

Value

As a security-consicous user, I want to see/leak as little metadata as possible.

As an admin who might want to temporarily let a slightly less trusted person send messages to the channel, I don't want to leak other admin's phone numbers to that person, because withholding that information (1) makes it harder for an infiltrator to maliciously hijack the channel, and (2) gain leverage for surveillance. Admins can thereby feel safer "sharing the keys" if they want to.

Behavior

GIVEN any channel

• WHEN an admin sends INFO
  • THEN they should receive a response that does not include other admin's phone numbers
• WHEN an admin is added
  • THEN the notification to other admins should not include the other admin's phone number
• WHEN a subscriber sends INFO
  • THEN they should receive a response that includes the number of subscribers on the list but NOT the number of admins