feature(client): Repopulate form fields after visiting the TOS/PP agreements.

fixes #1226
Replace `_savePrefillInfo` with the already built in `beforeDestroy`.

* No need to hook up _savePrefillInfo on every click handler!
Call `enableSubmitIfValid` from form.js->afterRender so subclasses do not have to do this themselves.

* Fixes a problem on signup where the "next" button is not enabled if all three fields filled out, the user clicks TOS, then "back."
Merge pull request #1278 from mozilla/issue-1226-remember-info-tos

feature(client): Repopulate form fields after visiting the TOS/PP agreements.