allow referring to threads by their full URL

[pabs3]

Currently the URLs on the ThreadTree service to threads are just the thread number https://threadtree.xyz/113155471560332059.

This means that if the ThreadTree service were to go down, and a user still has ThreadTree thread URLs in their browser history or other location, then there is no way to derive the original thread URL on Mastodon from the thread URL on the ThreadTree service, making it a bit harder to find the original Mastodon URL.

So it might be a good idea to append the original Mastodon URL to the ThreadTree service domain, giving URLs like https://threadtree.xyz/https://mastodon.social/@regehr/113155471094850763.

The existing ThreadTree service thread URLs could be turned into redirects to the new Mastodon URL based thread URLs.