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.

test/support/http_request_mock.ex

@@ -577,6 +577,16 @@ defmodule HttpRequestMock do
      }}
   end
 
+  # the same status via its canonical id
+  def get("http://mastodon.example.org/users/admin/statuses/99541947525187367", _, _, _) do
+    {:ok,
+     %Tesla.Env{
+       status: 200,
+       body: File.read!("test/fixtures/mastodon-note-object.json"),
+       headers: activitypub_object_headers()
+     }}
+  end
+
   def get("http://mastodon.example.org/@admin/99541947525187368", _, _, _) do
     {:ok,
      %Tesla.Env{