Figure out E2EE for state events.

The human-readable bits of state events (i.e. the bits that a redaction would apply to) should be encryptable using at least a static key, which could be shared around the room between clients. The same static key could be used for basic non-ratchet E2EE in massive rooms. Somehow we seem to lack a bug for this, so: here we are.

The issue names no files, tests, or entry points. Begin by reviewing the Matrix specification's state-event redaction and encryption sections, then determine how human-readable state-event fields and a shared static key would be specified. Done means the protocol behavior and key-sharing approach are defined for both ordinary rooms and massive rooms.

Written by the indexing model from the issue text.