Bug 1043533 - email UI: account settings styling

This changeset looks bigger than it actually is because of HTML whitespace indenting, so it may be better to review this using `?w=1` on the pull request URL to cut out those differences.

The whitespace changes come from using a `<ul class="tng-list">` to hold the li elements in the settings screens. It has bothered me they were free floating lis and by doing this, then we can target :last-child selectors to not show the bottom borders in those cases.

The settings_account.js is related to that change: instead of just hiding an element section that does not apply, the DOM element is removed.

I also removed some dead HTML in settings_main.html since it is not wired to anything, and it is more likely to cause confusion, be out of date HTML whenever we do have app-wide (and not account specific) user-configurable settings.