feature(client): Disable the submit button while auth server requests are outstanding.

* In Form based views, add beforeSubmit and afterSubmit. beforeSubmit disables form submissions until the submission completes and afterSubmit is called.

fixes #751
feature(client): Event non-form submissions are limited to one Fxa client request at a time.

* Convert all views that make fxaClient requests to FormViews.
* For sign_in.js->resetPasswordNow, handle form resubmission inline.
feature(client): Validating sessionToken on views that mustAuth.
refactor(views): move error translation to its own method
Merge pull request #810 from mozilla/issue-751-disable-submit

feature(client): Disable the submit button while auth server requests are outstanding.
Merge pull request #831 from mozilla/issue-743-validate-session-token

feature(client): Validating sessionToken on views that mustAuth.
fix(client): transition from password reset confirmation to signin once reset is complete