Clean up cross-signing events

The events that fire on cross-signing stuff changing are somewhat confusing and unclear. We should work them into a more consistent story.

Including, but not limited to:
* `userTrustStatusChanged` and `crossSigning.keysChanged` seem to occur in most of the same places, but not all the same ones, and have emitters duplicated in many places
* `_storeTrustedSelfKeys` emits that your own user trust has changed but should it emit events for any cross-signed user?
* Maybe there should be a specific event for when the value of `isCrossSigningReady()` changes, rather than the app having to listen for changes in the various things it checks.