Add support for authentication and authorization of user accessing instances

[craig-willis]

Problem/Approach:
See whole-tale/whole-tale#94

Prerequisites:

• Ask me for the v2 acme.json or download https://github.com/traefik/traefik-migration-tool, cd deploy-dev/traefik/acme and run sudo traefik-migration-tool acme to convert the acme.json to v2
• Requires this branch, Migrate to traefik v2 and add support for forward auth gwvolman#135, and Add endpoint to check whether user is authorized to access an instance girder_wholetale#454

Test case:

• Go to https://dashboard.local.wholetale.org
• Create a tale and start the instance
• Confirm that the instance can be accessed via iframe and pop-out tab
• Open the pop-out tab URL in a private/incognito session
• Confirm that you are redirected through the oauth flow and can access the instance
• Open the pop-out tab URL in a new private/incognito session for another user
• Confirm that you get a 403 error (Access denied for instance)

[Xarthisius]

That's no longer necessary.