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
fix(client): Change the "Next" button to say "Sign up"

fixes #1714
Merge pull request #1754 from mozilla/issue-1714-sign-up-button

fix(client): Change the "Next" button to say "Sign up"

Fixes #1714
r=ckarlof
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.
feat(test): Add more functional tests for the webChannel OAuth flows.