Copying zotero:// and other non-HTTP links from a note itself was fixed
in #452, but copying such links from other sources still stripped the
hrefs. This removes the patch in #452 and just gets the HTML directly
from the clipboard. I'm not sure why TinyMCE doesn't try to do this by
default (it only tries for plaintext), so maybe there's a problem with
this approach, but it seems to work for me (strips bad HTML, etc.).