notify inviter when invite accepted

value

admins have expressed that it is confusing that when they issue invites, they do not know who has accepted their invite.

as a member (admin | subscriber) of a signalboost channel, I'd like to know when someone accepted my invite and successfully joined the channel.

security note: we considered obscuring the phone number in the notification to the inviter upon "invite accepted." however, because we include the plaintext phone number in the "invite issued" command response, it does not provide any additional security in the case of phone seizure. whether we should make both of these notifications more ephemeral is considered in #361.

behavior

lola: admin | subscriber (should be the same behavior for both)
alice: admin | subscriber

zack: random person, hasn't interacted with signalboost yet

prior to new behavior

given a channel with VOUCHING OFF:

• when lola issues a valid invite to +14157775555

  • then she receives a response saying Successfully invited +14157775555.
  • then zack receives a message that says Hello! You have received an invite to join the [foobar] Signalboost channel. Please respond with ACCEPT or DECLINE.

• when alice issues a valid invite to +14157775555

  • then she receives a response saying Successfully invited +14157775555.

new behavior

• when zack responds with ACCEPT:
  • then lola receives a notification that says +14157775555 has accepted your invite, and is now subscribed to this channel.
  • then alice receives a notification that says +14157775555 has accepted your invite, and is now subscribed to this channel.

vouch levels

the behavior for channels with vouching ON/vouching ADMIN and varying vouch levels should remain the same; people who were successfully able to issue invites should get the same notfication as above when the invitee accepts their invite.

implementation

playing this card will involve:

• creating a new notification & subsequent translations in messages
• adding behavior in execute#accept to generate a notification to send to the inviter

Related: #342