Reapply "Remove jQuery from copy_as_gfm behavior", add regression tests, fix missed paste event

What does this MR do and why?

• Reapplies Remove jQuery from copy_as_gfm behavior (!232448 - merged) (reverted in Revert "Merge branch 'fix-issue-595359' into 'm... (!242650 - merged) due to breaking copy-as-GFM)
• Adds specs, including one which catches the bug introduced by it
• Fixes that bug (by reintroducing the paste event handler)
• Strengthens some other specs

References

• Closes Explore testing copy event with selection for c... (#604324).

How to set up and validate locally

Try copy and pasting some GFM-rendered content into a plain-text editor. The formatting should come with it, like this:

When it's broken, it looks like this:

MR acceptance checklist

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