Information on E2E key store state in the settings dialog

The current settings dialog doesn't give the user much information on the state of their E2EE key storage. I think we could show some basic information which could help the user understand the state of their account. For example:

* Is upload of new keys working?
* Is on-demand download of existing keys working?
* How many keys are known by the client?
* How many of those are waiting for upload?
* How many keys are stored on the server?

Some more advanced information which we might prefer to hide behind an "advanced" panel, but can still be useful:
* What is the key backup version on the server (and is that the same version that we are using for upload)
* If we're not using the key backup for upload, why not
* If we're not using the key backup for download, why not