fix a permission mismatch

[Alex-Jordan]

It seems to me that the wrong permission is referenced here. The problem editor page has nothing to do with student data.

What I'm changing it to seems like the best option. In some cases, the problem editor page allows you to edit a problem set. Although, if you enter it directly then it would seem that ability to edit a problem set is irrelevant, and why not let someone code a new problem? But this change is at least a better option than what is in there now, I think.

I came upon this reviewing old issues, in particular #746. For that issue to make sense, my guess is that modify_problem_sets is what should be here too. Assuming we don't make something better now that the problem editor is more general purpose than it once was.

[drgrice1]

Seems right to me.

[drgrice1]

Another possibility would be to use the modify_problem_template_files permission. That permission is currently not even used for anything.

[Alex-Jordan]

It seemed like a few other places there is logic based on modify_problem_sets giving access to the editor, although I did not do a thorough investigation. Unless you think modify_problem_template_files would be better here, I propose leaving this PR the way it is. But also I was going to separately do a comprehensive refactoring of what grants access to the editor. Both to address #746 and also to make it make more sense now that the editor has more uses than it once did. I haven't thought it through yet but I think it would make this current PR moot.

[drgrice1]

Another possibility is to use manage_course_files. Technically all files that can be edited are course files.

Also, this could depend on more than one permission.

In any case, this pull request is fine.