Change links in webUI to rewrite misleading links (#11426)

* [WiP] Show host for “misleading” links

* Disallow misleading targets which domain names are prefixes of link text

* Move decodeIDNA to app/javascript/mastodon/utils

* Add support for international domain names

* Change link origin tag color to darker text color

* Handle links to domains starting with www. as shortened by Mastodon

* [WiP] Ignore links that cannot be misread as URLs, rewrite other links