In order to sign and verify signatures of design keys, we need
to have them in the format as they are uploaded and downloaded
from the homeserver. So, like the cross-signing keys, we store
them in locally in the same format to avoid constant convertions.
I also renamed deviceIdentities to deviceKeys, analogue to
crossSigningKeys. In order to prevent mistakes in this refactor,
I also converted DeviceTracker to typescript.
