/webmention: allow any backlink, not just request.host_url

first step toward making local flask server fully capable of acting like fed.brid.gy

webmention.py

@@ -62,10 +62,11 @@ class Webmention(View):
 
         # check for backlink to bridgy fed (for webmention spec and to confirm
         # source's intent to federate to mastodon)
-        host_url = request.host_url.rstrip('/')  # don't require / path
-        if (host_url not in source_resp.text and
-            urllib.parse.quote(host_url, safe='') not in source_resp.text):
-            error(f"Couldn't find link to {host_url}")
+        for domain in common.DOMAINS:
+            if domain in source_resp.text:
+                break
+        else:
+            error(f"Couldn't find link to {request.host_url.rstrip('/')}")
 
         # convert source page to ActivityStreams
         entry = mf2util.find_first_entry(self.source_mf2, ['h-entry'])