Replacing user dropdown field with user search textbox. On stage due to large…

Replacing user dropdown field with user search textbox. On stage due to large user's list page is not loading.
ECOM-2980

lms/djangoapps/certificates/admin.py

@@ -52,6 +52,7 @@ class GeneratedCertificateAdmin(admin.ModelAdmin):
     """
     Django admin customizations for GeneratedCertificate model
     """
+    raw_id_fields = ('user',)
     search_fields = ('course_id', 'user__username')
     list_display = ('id', 'course_id', 'mode', 'user')