Group Community TA is added even though there are no divided discussions configured

Expected behavior

When adding a community TA to a course without cohorts and divided discussions enabled through the membership tab in the instructor dashboard, the Instructor should see an error, and the user shouldn't be assigned with role community TA.

Actual behavior

The error is displayed as expected, however:

1. User isn't shown in the community TA section in Membership, but it's seen in the discussions that user has Community TA status.

2. The user is added as Community TA to discussions

3. It isn't possible to revoke access until the divided discussions functionality is enabled for a course.

Steps to reproduce

• Setup discussions in the course without enabling Divide discussions by cohorts option
• Try to add user as Community TA on the Membership tab in the Instructor dashboard
• Check discussions post by the assigned user

Additional information

After the Cohorts are enabled for the course and Divide discussions by cohorts option is enabled in discussions, the user appears in the list of Community TA

This issue was reported as part of the redwood testing process: https://github.com/openedx/wg-build-test-release/issues/373

Start by reproducing the Membership-tab flow for adding a Community TA when cohorts and divided discussions are disabled, then trace how the assignment is reflected in discussions. Done means the failed operation leaves no Community TA role or discussion access, and access can be revoked without first enabling divided discussions.