Ensure all admin users stay unchecked after error.

When adding a new course, and someone unchecks all admin users to
be copied to the new course, ensure they stay unchecked if an error
occurs and the page is reloaded.

templates/ContentGenerator/CourseAdmin/add_course_form.html.ep

@@ -49,7 +49,7 @@
 				<div class="form-check">
 					<label class="form-check-label">
 						<%= check_box 'add-admin-users' => $user, class => 'form-check-input',
-							$val == 20 ? (checked => undef) : () =%>
+							$val == 20 && !param('last_page_was_add_course') ? (checked => undef) : () =%>
 						<%= "$user ($role)" %>
 					</label>
 				</div>
@@ -211,6 +211,7 @@
 			</label>
 		</div>
 	</fieldset>
-	<%= hidden_field add_dbLayout => 'sql_single' =%>
+	<%= hidden_field add_dbLayout             => 'sql_single' =%>
+	<%= hidden_field last_page_was_add_course => 1 =%>
 	<%= submit_button maketext('Add Course'), name => 'add_course', class => 'btn btn-primary' =%>
 <% end =%>