refactor(client): Separate `/force_auth` specialized code from `/sign_in` code.

* Add a helper function to session.js -> isSync
* Add a helper function to base.js -> BaseView.cancelEventThen

first step in #762
Merge pull request #935 from mozilla/issue-762-tease-out-force-auth

refactor(client): Separate `/force_auth` specialized code from `/sign_in` code.