Carol/docs/doc 1127

Moved "Bulk Email" into its own section under "Running Your Course" (was previously under Course Launch Activities in "Releasing Your Course"). Xrefs remain intact.
Removed note about bulk email being a limited feature per confirmation with Michele S. and Mark R. Changed wording of intro sentence to "Bulk Email" to indicate that this feature is only available for courses run on edx.org.
Added new email template "Weekly Highlights", which @mmacfarlane will review with course team (examples came from one of their emails).
@mhoeber @lamagnifica @srpearce 
can we take the opportunity to change this structure so it isn't so overloaded with "not"s?

Email messages are sent to all students with these exceptions.
* Students can opt out of receiving... (rest as it is)
* Email messages are sent only to students who have replied to the account activation email message that is sent when they register on edx.org. (this bullet might work better coming first, as it would naturally be the first check that would limit the recipient list)