replace channel/member phone number w/ uuid upon destruction event
context
- currently, we track creation/deltion of members and channels to be able to track growth over time
- since we keep these records indefinitely, and since we want users to be able to remove their data from our system once they leave, we use hashed/salted phone numbers so that when a channel or member is destroyed, it is impossible for an attacker who does not know the salt to recover the phone number (an attacker who does know the salt would have to brute force all possible phone number combinations to recover phone numbers from hashes, which is hard but possible)
- we would prefer it to be impossible for an attacker (or a signalboost maintainer) to be able to recover phone numbers of destroyed channels or memberships
- in this MR, we will generate a UUID and replace the (hashed) phone number with it upon channel / membership destruction
- this will preserve the property of allowing us to run stats on deleted channels (by uniquely identifying them)
- it will remove the property of it being possible for an attacker to recover deleted channel / member phone numbers by compromising our salt hash
- note: under this schema, a member who deletes their last membership and then rejoins will be double counted, but that seems preferable than keeping their phone number in a way that could be brute forced by an attacker who knows our hash salt!