Initial support for verification in right panel

[bwindels]

Requires matrix-org/matrix-js-sdk#1140
Part of element-hq/element-web#11224

Two big things happen in this PR (and the js-sdk PR):

• merge responsibilities (track state of request, drive UI) of KeyVerificationStateObserver into VerificationRequest (in the js-sdk), and attach that object to all the involved events as event.verificationRequest. Synchronizing the toast, event tiles and right panel is a lot easier this way. VerificationToast, MKeyVerificationRequest and MKeyVerificationConclusion needed to be ported to this.
• initial implementation of the verification right panel

I have tested that to_device verification still works, especially for verifying your own devices, as we might still need this for cross-signing.

Apologies that the commit log is a bit messy, might be better to review the changes in their entirety.

Cleanups still to do:

• fix lint errors
• remove debug logging
• move label functions in KeyVerificationStateObserver.js to a place that makes more sense now the class is removed there

[turt2live]

like the js-sdk, this is hard to follow but I think it's fine. It's slightly easier to follow than the js-sdk though, so there's that.

[turt2live]

I've also merged in develop here having broken the build system