[329] refactor `sdMessageOf`

* instead of taking unnamed `channel` and `mesageBody`
  take named `sender`, `recipient`, `message` and `attachments`
* motivation: simplifies the contract of `signal.sendMessage`, enables
  cleaner marshalling of `notification`s to be passed to
  `sendMessage`, is generally easier to read and clearer about the
  "outbound" cardinality of any and all sdMessages we ever construct