Bug 1068707 - improved accessibility of various email settings cards.

This (and all other checkboxes, switches) change is to be more consistent with what we did for the Settings app. In order to avoid using ID's the checkbox, switch, etc needs to be inside a label to have the appropriate relation (from the screen reader view). This also improves the way such elements are traversed: instead of landing on the label and the checkbox separately, the screen reader will land on the label and the screen reader user will be able to activate it through the label.