Enable E2EE by default

[ara4n]

This is a tracking issue for turning on E2EE by default. This means merging matrix-org/matrix-js-sdk#2002 and then using it for todevice signalling in element-call.

In practice, I think can go ahead and do this without further delay, because:

• I am confident that todevice messaging (at least between users on the same server) is robust on Synapse, having tortured it with https://github.com/ara4n/todevice-collider
• The rageshake reporting operates in terms of unencrypted messages, so we should be able to debug irrespective of E2EE assuming that to-device is working okay.
• Even if we do see to-device messages disappearing on the server, we can chase them through as opaque blobs anyway.

However, it's worth noting that E2EE is useless without verification. As a first cut we could do TOFU and persist the verification state in localstorage, but obviously this will not help us with a preexisting attack or impersonator, which would be pretty easy, not least given we're currently spinning up new user IDs everywhere rather than inheriting existing trust. I'll file a separate bug for that.