Bug 827575 Don't change newsletter dates

Don't change newsletter timestamps in ET when we're not changing the
user's subscription. Change Basket to not send subscribe for
newseltters already subscribed to, or unsubscribe for newsletters not
subscribed to.
Merge pull request #39 from dpoirier/bug-827575-dont-change-newsletter-timestamp-unnecessarily

Bug 827575 Don't change newsletter dates