fix(fetcher): require fetched documents to have the fetched id

The fetcher accepted any document whose id was on the same host as the
fetch URL. Anyone who can place a JSON file on a host, for example
through an upload, could pass it off as another object or actor from
that host.

The id of a fetched document must now be the URL it came from (after
redirects), ignoring a trailing slash, the fragment, the host case and
differences that our URL encoding removes.
If it differs, as for Mastodon's /@user/123 display URLs, we refetch
once from that id, which must be on the same host. The refetch must
not redirect and must match exactly.

Test mocks that served a document under a different URL now serve it
from its own id.

Based on Akkoma 8684964c5 and 940792f8b.

changelog.d/fetch-id-containment.security

@@ -0,0 +1 @@
+Only accept fetched objects whose id is the URL they were fetched from; for other ids, refetch once from the canonical id on the same host