0.2.0-pre: upgrade to ww-rs-eta 0.5.0-pre1

- We now use the fancy new improved WhatsApp connection logic
  in ww-rs-eta 0.5.0-pre1, which is nice.
- As a result, the handling of failed and superseded connections
  should be vastly improved; no more random QR code requests from
  lingering connections!
- In addition, the user now gets notified whenever they *remove*
  the connection from their mobile app (although it could also
  conceivably remove the state from the DB as well)
- History fetching was removed, as it never worked.
- The `ignore_other_libraries` config key now doesn't ignore
  whatsappweb_eta, because it's useful to have that in there
  for debugging as well.
- Some `dyn`s were dotted about the place to satisfy Rust 2018 lints.