Update activation email text

Make it more active.  Adds a separate section for edX.org-specific copy,
because it's hard to make it generic.


tbh I'm surprised we're not including some version of this string in the edx-specific section. This is a standard qualifier in messages that may be unsolicited from a company. If I were you I'd figure out who to ask about this type of thing - we really don't want to be reported as spam.