Only block domains at the final destination (#15689)

In an earlier PR, we decided that we only want to block a domain if 
the blocked domain in the SiteSetting is the final destination (/t/59305). That 
PR used `FinalDestination#get`. `resolve` however is used several places
 but blocks domains along the redirect chain when certain options are provided.

This commit changes the default options for `resolve` to not do that. Existing
users of `FinalDestination#resolve` are
- `Oneboxer#external_onebox`
- our onebox helper `fetch_html_doc`, which is used in amazon, standard embed 
and youtube
  - these folks already go through `Oneboxer#external_onebox` which already
  blocks correctly

lib/onebox/domain_checker.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Onebox
+  class DomainChecker
+    def self.is_blocked?(hostname)
+      SiteSetting.blocked_onebox_domains&.split('|').any? do |blocked|
+        hostname == blocked || hostname.end_with?(".#{blocked}")
+      end
+    end
+  end
+end