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.


This is a deliberately edX-specific version of the email, inside a PLATFORM_NAME is "edX" block.  Do you still think there's value in variable-izing it?  I'm not sure we already have settings for all of these (e.g. the contact-us page).