Needs to support field experiments with a large number of users

[billpugh]

The app should support field experiments of GAEN with a large number of users: at least 10, maybe more than a hundred. To do this, exchanging keys via email/airdrop isn't feasible. Instead, we will need to use a server to do so. We can use something based on the reference server designs.

We will also want to collect the exposure reports via a server to allow them to be analyzed centrally.

We might want to extend that in a way that allows us to slightly abuse the protocol so that we can identify the individual responsible for each exposure, so we can match that to participants in a scripted or unscripted encounter. By using transmission risk and a batch number, we could distinguish up to 120 different participants.