don't show group discussion option for group level discussions

fixes CNVS-15050

test plan
- create a group
- ensure that when creating a discussion in the group, an 
 option is not shown to make it a group discussion
- ensure that when creating a discussion in a course, the
 option to make it a group discussion is shown

Change-Id: I8aa173529dc7ab39071115a3530b5fca93465035
Reviewed-on: https://gerrit.instructure.com/45578
Reviewed-by: Andrew Butterfield <abutterfield@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>