Bug 966150: adjust wording/layout of new-sync preferences pane, r=ttaubert

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extra : rebase_source : 2736e75660a0def3561070f8f5e4d50f9de1e5cf
Bug 966134: merge warning dialog should mention new email, not old email, and we should avoid storing the plaintext of signed-in users forever, r=markh

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extra : rebase_source : cee71c3201ee6fffc477d2cced50e0df567a0976