Reactions: Encrypt aggregation key in E2E rooms

[jryans]

The aggregation key / reaction value should be encrypted using the message key of the original event being reacted to.

[jryans]

This is no longer required for shipping reactions. We plan to add metadata to the encrypted aggregation keys so that it's clear to the client when they do need decryption.

[turt2live]

This is a spec issue, for which I believe an issue already exists (or at least a comment on the STILL unmerged MSC)