Panoptes.js: improve initial experimental auth

[shaunanoordin]

PR Overview

This PR improves PR #6375 with the following updates:

• Output & "error handling" for Event Listener functions:
  • addEventListener() no longer throws errors.
  • addEventListener() and removeEventListener() now returns true/false if the action succeeded/failed.
• Documentation: the Sign In and Event Listener functions now have full documentation.
• Code cleanup: unnecessary PJC reference code removed.
• Dev Server/Dev Test Page:
  • Dev test page now displays success and/or error messages on the web page itself, so we don't need to look at the dev console to see wtf is happening.

Status

Merging into 6375.