[498] refactor SignalReceiver code, add tests

in `SignalReceiver`:
- reorganize execution flow in `#dispatch` to be more cohesively grouped
- add `numberOfMessagesWithoutProfileKey` counter to track messages
  received from desktop app without "continue" being tapped from phone
- move `null` return up into `dispatch` (where it more obviously
  refers to whether or not a job is returned from a branch of the
  `when` statement and the helpers that don't return a job have a more
  sensible signature of `Unit`)
- in `#processPreKeyBundle`: remove unnecessary `withContext` wrapper
  and touch up comments
- in `#processReceipt` remove unnecessary check for
  `isSealedSender` (receipts are always unsealed) and touch up comments

in `ContactStore`:
- overload `#create` to take an account and envelope to make for more
  ergonomic calling from `SignalReceiver`

re: `SignalSender`
- add tests for `#sendProfileKey` and `#sendReceipt` helpers used by
  `SignalReceiver`