always store them, if not we'll think that the user hasn't uploaded
the cross-signing keys if we don't store them in spite of invalid or
missing signature.
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.
