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