Add Groups button no longer visible to students

fixes CNVS-15199

Added STUDENT_CAN_ORGANIZE_GROUPS_FOR_COURSE to ENV for check to see if students
are allowed to create groups.

Test Plan
- Allow "New Student Groups Page" in Feature Options tab of Account Settings
- Disable "Let students organize their own groups" in a course
- As a student, visit Groups page and notice that the "+ Group" button
  is hidden

Change-Id: I1379834ad44c0cedb13a41ec636547d4c3ffa1bb
Reviewed-on: https://gerrit.instructure.com/43259
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>