WIP: online incremental megolm backups

[ara4n]

This doesn't remotely work yet (not even tried to compile it yet) and isn't finished, but opening a PR to aid feedback/review

Adds a new crypto.suggestKeyRestore event which the js-sdk fires when the user starts interacting with e2e (by receiving room_keys) for the first time on a given device. The event should prompt the user to sync in keys.

Stuff left to do:

• Implementing the client side of the new E2E backup APIs
• Actually wire up the backupKey field to olm::Account
• In future, hook up the cross-signing field to /devices store
• Persist suggestedKeyRestore somewhere in localStorage or equivalent