Support pasting rich text link on description editor

[r7kamura]

I would like to see the issue and comment editors properly convert rich text links to Markdown format links when pasted ...or at least prevent oddly formatted strings from being pasted.

Example

I created a simple JavaScript to copy a link in both HTML and plain text format:

function copy({ html, plain }) {
  const listener = (event) => {
    event.clipboardData.setData("text/html", html);
    event.clipboardData.setData("text/plain", plain);
    event.preventDefault();
  };
  document.addEventListener("copy", listener);
  document.execCommand("copy");
  document.removeEventListener("copy", listener);
}

copy({
  html: '<a href="http://example.com/">foo</a>',
  plain: "foo bar"
})

We can try this by pasting the above code to devtools console.

When we try to paste the copied data into the editor on GitHub Issues, the following string is pasted:

[foo](http://example.com/) bar

...but, isn't this a little odd?

Ideally, I would like the following string pasted:

[foo bar](http://example.com/)

Or, less ideally, I would like to see the following plain text pasted:

foo bar