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Add button to create threadmark at top of post
Adds button (⭐) to top-right corner of posts to create a threadmark, same as the "Add threadmark" link at the bottom of a post.
Requires the ability to create threadmarks, which is usually granted to the user that creates a thread, thread collaborators, etc.
If user does not have the ability to create threadmarks, clicking the button is harmless but will do nothing except display an error message.
(Credits: uses code adapted from Vannius's userscript.)
