[FEATURE REQUEST] Team option to hide members' identity.

I'd like to build a team, where the members don't need to know each other.

For non-admin, the member list would be empty, or only show a number.
Potentially, people that follow each-others could see each other's identity.

when someone join a team, an automatically generated unique ID would be generated to track his messages are from the same person.
when people write a message, or when someone write `@username` that ID would be shown instead.

people can have a conversations, without knowing who are on the other side. but we could make it so they are able to know if that person has proven. aka showing month/year of sign up and non-exact number of followers would give enough of a clue, if this is brand new account, or someone who he is recognize has real, although we don't need to know exactly who he is.

I think that would be a great feature, that pretty much only keybase can achieve and that no-one has done before.

No files, tests, or entry points are identified. Start by mapping team membership visibility, member-list behavior, message authors and @mentions, and the proposed generated member IDs; the work is done only when the privacy rules and behavior for admins, non-admins, followers, and identity proofs are specified and tested.

Written by the indexing model from the issue text.