refactor(client): Move many parameters/helpers from Session to Relier.

@nchapman and @zaach - this is part one in the ["resume" PR](https://github.com/mozilla/fxa-content-server/issues/1623). The PR became pretty burly, so I factored out this portion, and will issue a follow on PR with the resume functionality.

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parameters:

* service
* most oauth parameters
* redirectTo
* context

helper functions:

* isSync
* isFxDesktop

Other work:

* Split of the OAuthRelier from the normal Relier
* Initialize Channels with the Relier in app-start
* Update the oauth-reset-password functional test to not hard code the email format. Instead fetch the link from the email headers.

Remove all of the resume stuff.

issue #1648
Any harm in just requiring that translator always be passed in here rather than ever relying on `_window`?