Don't revert to Advanced Editor for pasted problems

[pomegranited]

When editing a Problem component that was pasted into a Content Library, the Advanced Editor is being shown. Instead, show the problem-type-specific editor.

Issue was reproduced this on tagging-preview (see e.g. "Multiple Choice" problem in lib:OpenCraft:TL).

[bradenmacdonald]

@pomegranited The problem is the markdown and url_name fields. When I removed them from that example block, it started working.

We need to fix the paste code so it strips out the url_name attribute on paste. As for markdown, I'm not sure why it's there but I suspect if it was there, the MFE editor wouldn't have been working in the course that you copied it from either.