feat(client): Create and send a resume token to the OAuth server.

The `resume` token isn't actually used at the moment, this is setting us up to verify in a second browser. The resume token only contains the `state` token passed by the OAuth RP.

fixes #1623
Rename "toResumeToken" to "getResumeToken". Always send the resume token on signup, even if using a preVerifyToken.
Merge pull request #1646 from mozilla/issue-1623-resume-to-relier

feat(client): Forward the `resume` token to the auth server

@ckarlof is happy and gave the r+. Travis is happy and shows green. I'm happy.

Merging.