Encryption for message relationships

[Event relationships](#2674) are currently sent unencrypted so that they may be aggregated server-side. However, they may contain information that we do not want to be plain-text (e.g. we don't want the server to know if a user reacted with a :+1: or :-1: ). We should figure out a way to encrypt them. matrix-org/matrix-spec-proposals#1849 includes some [thoughts on how to do encryption](https://github.com/matrix-org/matrix-doc/blob/matthew/msc1849/proposals/1849-aggregations.md#thoughts-on-encrypting-the-aggregation-key), and sorunome and deepbluev7 have also given some thoughts at https://github.com/matrix-org/matrix-doc/pull/1849#discussion_r446515510

Start by reading the event relationships issue (#2674) and the encryption discussion in matrix-org/matrix-spec-proposals#1849, including the linked aggregation-key discussion. The issue does not identify a target specification file or test. Done would require an agreed encryption approach that is reflected in the Matrix specification.

Written by the indexing model from the issue text.