Disable fetching of root posts for relayables

since that could allow fetching spoofed/altered posts
thanks @supertux88

Changelog.md

 # 0.5.7.1
 
+This security release disables post fetching for relayables. Due to an insecure implementation, fetching of root posts for relayables could allow an attacker to distribute malicious/spoofed/modified posts for any person.
+
+Disabling the fetching will make the current federation a bit less reliable, but for a hotfix, this is the best solution. We will re-enable the fetching in 0.6.0.0 when we moved out the federation into its own library and are able to implement further validation during fetches.
+
 # 0.5.7.0
 
 ## Refactor

lib/federated/relayable.rb

@@ -40,7 +40,7 @@ module Federated
     end
 
     def fetch_parent guid
-      Diaspora::Fetcher::Single.find_or_fetch_from_remote guid, diaspora_handle
+      raise Diaspora::PostNotFetchable
     end
   end
 end