feat: Add support for the iframed oauth flow.

Set 123done up to serve either the iframe flow or the standard oauth flow.

Uses the fxa-relier-client

browse to `/` to use the normal flow.
browser to `/iframe` to use the iframe flow.
Merge pull request #125 from mozilla/iframed-oauth

Iframed oauth
Use the newest fxa-relier-client.
Merge pull request #126 from mozilla/newest-fxa-relier-client

Use the newest fxa-relier-client.
refactor(client): Fetch the fxa-relier-client from bower instead of storing it in the repo.

fixes #127
Merge pull request #128 from mozilla/issue-127-fxa-relier-client-bower

refactor(client): Fetch the fxa-relier-client from bower instead of storing it in the repo.
fix: Fix the redirect in the redirect flow.

In the redirect flow, the referrer is FxA, not 123done. This is bad jiji, because we were redirecting back to FxA.
Merge pull request #129 from mozilla/fix-the-redirect-flow

fix: Fix the redirect in the redirect flow.
load config from \$CONFIG_123DONE in the environment, if the file exists