Pass `primaryIdentityPublicKeys.ed25519`, `signedIdentityKeysBlob.payload` and `signedIdentityKeysBlob.signature` to `olmUtil.ed25519_verify(...)` to ensure that the `signature` is valid.
The reason this diff is titled `[DRAFT]` is because of the way we're calling `olm.init()` and `new olm.Utility()` every time `logInResponder` encounters a request with `signedIdentityKeysBlob`. Instead we'll want to probably run `olm.init()` once when `keyserver` starts up, and we'll want to instantiate `olm.Utility()` once and make it accessible globally.
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Depends on D6896