Panoptes.js Experimental Auth: add checkCurrentUser() aka checkCurrent()

[shaunanoordin]

PR Overview

Package: Package: lib-panoptes-js
Part of: replacing PJC with Panoptes JS
Follows and requires: #6375

This PR continues the experiment to remove PJC from FEM. In this PR, we add the ability to check the current user, via checkCurrentUser(), to the Experimental Auth system.

• Experimental Auth: added checkCurrentUser() (and its alias for backwards compatibility, checkCurrent() )
  • checkCurrentUser() will return the current signed-in User if the Panoptes API recognises there's one, based on the http-only Panoptes session cookies.
  • In the context of FEM, this is pretty much always used at the start of app load, start of page load, and/or the start of API function calls.
• Dev server: new "check current user" button added, to manually trigger the action.

Status

Ready for internal team review.

Please note that the code is verbose on purpose. I can fold a lot of the functionality into reusable functions (e.g. checkBearerToken() / fetchBearerToken() ) but I want to make sure that everyone on the team can read through & understand the process as easily as possible.