Unable to edit user overrides if assignment is not available to student

This issue was already detailed and fixed in MDL-51189 for quizzes, but it should also be applied to assignments (and any other activities that behave similarly).

To replicate this:

1. Add a lesson or assignment to a course
2. Hide the activity
3. From activity settings, select 'User override' and add an override
4. Notice you only have the controls to delete the override. There should be edit and copy controls there too.

Once fixed, we should consider updating MDLQA-10385 and MDLQA-7647 to test this hidden case.