Consensus on server-side data join

[benjaminsavage]

At the last face-to-face PAT-CG call, we discussed this design dimension: the location of data join (between source and trigger events).

There was consensus that the group was potentially open to data join happening server-side, so long as the system was able to achieve our desired privacy and security goals.

[AramZS]

As discussed, these PRs have been open long enough to acquire consensus that they accurately describe our conversations.