Improve searching for private toots from URL

Improve searching for private toots from URL

Most of the time, when sharing toots, people use the toot URL rather than
the toot URI, which makes sense since it is the user-facing URL.

In Mastodon's case, the URL and URI are different, and Mastodon does not
have an index on URL, which means searching a private toot by URL is done
with a slow query that will only succeed for very recent toots.

This change gets rid of the slow query, and attempts to guess the URI from
URL instead, as Mastodon's are predictable.