refactor(client): FxaClient no longer takes a relier in its constructor.

* Instead of creating the FxaClient with a relier in its constructor, pass a relier instance to the functions that need it.
* Since we had to update any unit test that used fxaClient.signUp to pass in a relier, it was easy to update the tests to no longer touch the server.
* FxaClient.signUp no longer calls .signIn, the view takes care of this.

fixes #1777
Merge pull request #1778 from mozilla/issue-1777-fxa-client-sans-relier

refactor(client): FxaClient no longer takes a relier in its constructor.

Everyone is giving it the thumbs up, I'm merging it.