AA-142 calendar sync ses

- set up util function to use Amazon SES for sending calendar
  sync emails.
- remove old sailthru code which we originally implemented for
  this functionality
- include ADR on our decision to use SES instead of Sailthru.

openedx/features/calendar_sync/docs/decisions/0001-calendar-sync-emails-using-ses.rst

@@ -0,0 +1,24 @@
+1. Amazon SES for calendar sync emails
+================================================
+
+Status
+------
+
+Proposed
+
+Context
+-------
+
+For calendar sync functionality, we would like to send users
+emails with .ics file attachments, which will in turn allow the user
+to easily add/update course deadline dates on their personal calendars.
+
+Decision
+--------
+
+We will use Amazon SES to send these emails.  Originally, we had hoped to use
+Sailthru, but found that file attachments were not supported.  While emails
+with attachments are not currently sent from platform at the time this doc is
+being written, they are however sent from other services such as
+enterprise-data using Amazon SES.  We will use a similar approach for our
+calendar sync feature.