feat(client): Add support for the `resume` token.

This build on #1645 and should be reviewed afterwards.

* `resume` is imported from the URL and stored in the Relier.
* fxa-client sends the token to the auth server in `signUp`, `signUpResend`, `passwordReset`, and `passwordResetResend`

issue #1623
Added the `encodeURIComponent` at @pdehaan's suggestion.