feat(client): Smooth out the verification flow.

* The original tab always automatically completes the oauth flow if it is open when the email is verified.
* If a second tab is opened in the same browser, show the user a link to continue back to the RP.
* If the user verifies in a second browser, show the service name, but do not allow the user to click back to the RP.

fixes #1723
In light of https://github.com/mozilla/fxa-content-server/pull/1725#issuecomment-57560872, I'm not sure what the right thing to do here is. Should we clear the oauth data?