Upload Courses tool breaks on locked custom fields

Steps to reproduce:

1. Visit /course/customfield.php. Create a new custom field.
2. Edit the custom field, set "Locked" = "Yes".
3. Visit /admin/tool/uploadcourse/index.php
4. Upload a simple course CSV, e.g:

5. shortname,fullname,category

   test1,Test 1,1

1. Click "Preview". You should get an error: "Coding error detected, it must be fixed by a programmer: Only course or course category context expected"

This error is caused by incorrect assumptions in /course/classes/customfield/course_handler.php, in the can_edit function:

return (!$field->get_configdata_property('locked') || guess_if_creator_will_have_course_capability('moodle/course:changelockedcustomfields', $context));

When uploading a course, $context will always be an instance of context_system, because the caller passes $instanceid = 0. The function guess_if_creator_will_have_course_capability does not allow $context to be a context_system instance. So if the field in question is locked, that function is called with a context_system instance and a coding error is thrown.