feat: use feature setting for beta tester notify

[mohd-akram]

Description

Use the BATCH_ENROLLMENT_NOTIFY_USERS_DEFAULT feature setting for the default value of the "notify users by email" checkbox under Batch Beta Tester Addition in a course's instructor dashboard (membership tab).

Testing instructions

1. Set BATCH_ENROLLMENT_NOTIFY_USERS_DEFAULT to True
2. Ensure the notify users checkbox is ticked by default
3. Set BATCH_ENROLLMENT_NOTIFY_USERS_DEFAULT to False
4. Ensure the notify users checkbox is unticked by default

Private-ref: BB-7751

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[Agrendalath]

👍

• I tested this: checked that the behavior of the "Notify users email" checkbox in the "Batch Beta Tester Addition" section matches other checkboxes on this page
• I read through the code
• I checked for accessibility issues: n/a
• Includes documentation: n/a
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository: n/a

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[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.