Make ENABLE_S3_GRADE_DOWNLOADS disabled by default, enabled in dev.

Having ENABLE_S3_GRADE_DOWNLOADS enabled by default in common.py
could lead to surprising behavior for folks downstream. They'd
suddenly see a grade download screen on their new instructor
dashboard, but the links by default would be local files and
couldn't be used in an actual production environment. So we disable
by default and let people explicitly enable it and set it up for
S3 if they wish.

LMS-58

CHANGELOG.rst

@@ -5,6 +5,10 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+LMS: Add feature for providing background grade report generation via Celery
+  instructor task, with reports uploaded to S3. Feature is visible on the beta
+  instructor dashboard. LMS-58
+
 LMS: Add a user-visible alert modal when a forums AJAX request fails.
 
 Blades: Add template for checkboxes response to studio. BLD-193.

lms/envs/common.py

@@ -195,7 +195,8 @@ MITX_FEATURES = {
 
     # Grade calculation started from the new instructor dashboard will write
     # grades CSV files to S3 and give links for downloads.
-    'ENABLE_S3_GRADE_DOWNLOADS': True,
+    'ENABLE_S3_GRADE_DOWNLOADS': False,
+
     # Give course staff unrestricted access to grade downloads (if set to False,
     # only edX superusers can perform the downloads)
     'ALLOW_COURSE_STAFF_GRADE_DOWNLOADS': False,

lms/envs/dev.py

@@ -40,6 +40,7 @@ MITX_FEATURES['ENABLE_INSTRUCTOR_BETA_DASHBOARD'] = True
 MITX_FEATURES['MULTIPLE_ENROLLMENT_ROLES'] = True
 MITX_FEATURES['ENABLE_SHOPPING_CART'] = True
 MITX_FEATURES['AUTOMATIC_VERIFY_STUDENT_IDENTITY_FOR_TESTING'] = True
+MITX_FEATURES['ENABLE_S3_GRADE_DOWNLOADS'] = True
 
 FEEDBACK_SUBMISSION_EMAIL = "dummy@example.com"