Add option to convert twitter links to xcancel links.

[r3v]

Some servers would prefer not to drive traffic to twitter/x. In the website settings, it could be a simple toggle next to the others. If enabled, it would change the links from x.com to xcancel.com.

[Kyrela]

It's already possible. You can:

1. Disable the Twitter fixes, and register xcancel.com as a custom website (It'll use xcancel embeds and link, for everyone)
2. Follow the instruction at FxTwitter (It'll still use FxTwitter embed, but the link will redirect to the configured Nitter instance, for the persons who configured it).