Better diagnostics for key backup key inconsistency in E2EE dev tools

We have observed that some users appear to have inconsistent key backup keys.

Specifically: their locally-cached decryption key is inconsistent with the published encryption key. We are unsure if this is because their local decryption key is inconsistent with the local encryption key, the local encryption key is inconsistent with the published encryption key, or both.

To address this, it would be helpful to add more diagnostics to the Encryption devtools. For example:
* Display the current server-side backup version number, rather than a cached version as it currently does.
* Display the server-side and local-side public (encryption) keys.
* Check that the local public and private keys are consistent, and show a fat red error if not.