Signs and verifies frames. For now this is done on every frame.
This is not the long-term goal and there needs to be a better strategy.

this uses the full four bits in the wire format. The wire format
looses the (currently not implemented) extensibility with variable
length keys.

similar to what is explained here:
  https://tools.ietf.org/html/draft-omara-sframe-00#section-4.3.5.1
but we do it on authentication tag failures since it is not possible
to tell whether decrypt.

following
  https://tools.ietf.org/html/draft-omara-sframe-00
but putting the frame metadata into a trailer instead of a header.
We call this JFrame.

Also the key we get from OLM is high entropy so we do not need
to use PBKDF2 but can use HKDF instead. See
  https://wiki.developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/deriveKey#HKDF

derived from the participant id in addition to the salt, separated by a null
byte to avoid ambiguity attacks along the lines of
  (someRoom, someParticipant)
  (someRoo, mSomeParticipant)

Moveѕ e2ee operations to a worker that is included as text/blob for now
to simplify deployment.

fixing some minor issues and updating the description of the behaviour
when there are decoding errors

adds a document describing the high-level design of the current e2ee approach