Use iso8601 Ruby date method for authentication token

[camdendotlol]

The authentication controller creates a date string for the token, which the frontend JS parses to determine whether the token is expired.

The format used for the token is parseable in Chrome's JS engine, but not in Firefox. If you replace the strftime method with iso8601, the resulting string will be in a format guaranteed to be parseable in all JS engines.

https://github.com/performant-software/iiif-cloud/blob/master/app/controllers/api/authentication_controller.rb#L27

[camdendotlol]

This issue also affects Safari.