feat: allow disabling default beta tester enrollment notifications

The "Notify users by email" checkbox checks the value of the 
`BATCH_ENROLLMENT_NOTIFY_USERS_DEFAULT` configuration to match the user 
experience with other checkboxes on the Membership page.

lms/templates/instructor/instructor_dashboard_2/membership.html

@@ -118,7 +118,8 @@ <h3 class="hd hd-3">${_("Register/Enroll Students")}</h3>
     </div>
     <div class="enroll-option">
         <label class="has-hint">
-            <input type="checkbox" name="email-students-beta" id="email-students-beta" value="Notify-students-by-email" checked="yes" aria-describedby="heading-batch-beta-testers">
+            <input type="checkbox" name="email-students-beta" id="email-students-beta" value="Notify-students-by-email" aria-describedby="heading-batch-beta-testers"
+                   ${'checked="yes"' if settings.FEATURES.get('BATCH_ENROLLMENT_NOTIFY_USERS_DEFAULT') else ''}>
             <span class="label-text">${_("Notify users by email")}</span>
             <div class="hint email-students-beta-hint">
                 <span class="hint-caret"></span>