Enable more markdown paste features in textarea editor (#35494)

Enable the [same paste
features](https://github.com/github/paste-markdown#paste-markdown-objects)
that GitHub has, notably the ability to paste text containing HTML links
and have them automatically turn into Markdown links. As far as I can
tell, previous paste features all work as expected.

---------

Signed-off-by: silverwind <me@silverwind.io>

web_src/js/utils/url.ts

@@ -2,18 +2,6 @@ export function pathEscapeSegments(s: string): string {
   return s.split('/').map(encodeURIComponent).join('/');
 }
 
-function stripSlash(url: string): string {
-  return url.endsWith('/') ? url.slice(0, -1) : url;
-}
-
-export function isUrl(url: string): boolean {
-  try {
-    return stripSlash((new URL(url).href)).trim() === stripSlash(url).trim();
-  } catch {
-    return false;
-  }
-}
-
 /** Convert an absolute or relative URL to an absolute URL with the current origin. It only
  *  processes absolute HTTP/HTTPS URLs or relative URLs like '/xxx' or '//host/xxx'. */
 export function toOriginUrl(urlStr: string) {