webfinger: handle Bridgy Fed actor URLs

hoping to fix errors like this on Mastodon AP delivery:
`Received 401: Public key not found for key https://fed.brid.gy/snarfed.org`

tests/test_webfinger.py

@@ -185,7 +185,8 @@ class WebfingerTest(testutil.TestCase):
         mock_get.return_value = requests_response(self.html, url='https://foo.com/')
 
         for resource in ('foo.com@foo.com', 'acct:foo.com@foo.com', 'xyz@foo.com',
-                         'foo.com', 'http://foo.com/', 'https://foo.com/'):
+                         'foo.com', 'http://foo.com/', 'https://foo.com/',
+                         'http://localhost/foo.com'):
             url = '/.well-known/webfinger?%s' % urllib.parse.urlencode(
                 {'resource': resource})
             got = self.client.get(url, headers={'Accept': 'application/json'})

webfinger.py

@@ -162,6 +162,10 @@ class Webfinger(Actor):
     """
     def template_vars(self):
         resource = flask_util.get_required_param('resource')
+
+        # handle Bridgy Fed actor URLs, eg https://fed.brid.gy/snarfed.org
+        resource = resource.removeprefix(request.host_url)
+
         try:
             user, domain = util.parse_acct_uri(resource)
             if domain in common.DOMAINS: