Option to expose auth bundle to kernel gateway

[parente]

The auth passportjs strategies currently store arbitrary metadata in the express session. In some use cases, the kernel may need information from this bundle in order to act on behalf of the authenticated user. For example, if Dropbox is the configured OAuth dashboard provider, and a dashboard / notebook wants to read/write data from the authenticated dashboard user's Dropbox account, the scoped Dropbox access token granted privileges by the user at login needs to be shared with the kernel.

The kernel gateway has the ability to receive environment variables prefixed with KERNEL_ on a POST /api/kernels request and set these in the environment of a newly launched kernel. An option here in the dashboard server can take advantage of this to:

1. Grab the user object from the passport strategy done(null, user) call.
2. Pass it along as a JSON value for a KERNEL_USER_AUTH env var in the POST request.
3. Write notebooks that read the env var, parse the JSON, and do what they want.

The option on the dashboard server in config.json can be:

// Forward auth strategy user profile to every kernel launched for every dashboard visited on the server
// e.g., so the kernel can re-use an auth token to access an external service
KG_FORWARD_USER_AUTH: false

[parente]

Related to some of the comments in jupyter/dashboards#13. We've got some of the security licked already. This issue is about sharing of creds between frontend login and backend kernel.

/cc @lmeyerov