[#295] Resolve "display admin id with command notifications"

context

Now that we have admin id's, we should use them to inform other admins about what is happening on a channel!

changes

notifications in execute

• adds admin id's to the following notifications (& changes to their translation accordingly):
  • admin added
  • admin removed
  • hotline on/off
  • vouching mode
  • vouch level changed
  • admin leaving
• creates addNotificationHeader() in execute

notifications in destroy

• in registrar#destroy we now distinguish between the type of "issuer":
  • ADMIN which gets a channelDestroyed notification
  • SYSTEM 🤖 which gets a channelDestroyedDuetoInactivity notification

refactor

• manual QA revealed that sometimes the adminId was undefined because we weren't passing in sender as a valid membership
  • so we refactored dispatcher#classifyPhoneNumber to return a sender membership
• we also refactored execute#addAdminIdToHeader to take advantage of that sender membership.

Closes #295 (closed)