Run parity if installed

Ghost User requested to merge am-run-parity into next

This PR does, on UI launch, the follwing

  • if parity running, do nothing
  • if parity installed and not running, run parity
  • if parity not installed, show download instructions

To test:

  1. Open UI with parity running -> Shoudl work as before
  2. Don't run parity and open UI -> Should spawn a parity instance (i.e. ps aux | grep parity should list parity). Note: since parity is run as a child process, closing the UI will close parity as well.
  3. sudo mv /path/to/parity /path/to/parity.bak, launch UI -> Should show download instructions

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