Switch xcancel with xfixup

[theetrain]

We currently respond to posts containing x.com links with a converted xcancel.com link. Since xfixup.com links contain nicer embed previews, let's switch to that.

[theetrain]

Note, redirects may be necessary otherwise whole threads won't be visible from the default redirect to x.com. Details: https://github.com/FxEmbed/FxEmbed?tab=readme-ov-file#redirect-to-nitter-or-other-custom-instances-xtwitter.