CG-1904 improve expired session ux

[carl-alexander-adams]

• Improve the user experience around old stale sessions that appear to be
  initialized, but are actually expired. This is done by providing the new
  utility method Auth.ensure_request_authenticator_is_ready().
• Save computed expiration time and issued time in token files. This allows
  for the persistence of this information when dealing with opaque tokens.
• Support non-expiring tokens.

[tbarsballe]

I'd like to see the backwards-compatibility I noted, but I don't think that's strictly necessary - let me know what you think and I could change this to approved

[tbarsballe]

Following on from discussion, it appears my concerns around backwards compatibility are not applicable, as expiry is parsed from the JWT upon initial load.

As such, marking this as approved (though there is still the one comment that you could fix or not)

[carl-adams-planet]

Following on from discussion, it appears my concerns around backwards compatibility are not applicable, as expiry is parsed from the JWT upon initial load.

As such, marking this as approved (though there is still the one comment that you could fix or not)

I'll still go ahead and put a note in the change log. on the one in a billion chance someone used this with an opaque token somewhere.