Return salmon path for users, basic incoming salmon handling.

test/web/ostatus/feed_representer_test.exs

@@ -27,6 +27,7 @@ defmodule Pleroma.Web.OStatus.FeedRepresenterTest do
       <title>#{user.nickname}'s timeline</title>
       <updated>#{most_recent_update}</updated>
       <link rel="hub" href="#{OStatus.pubsub_path(user)}" />
+      <link rel="salmon" href="#{OStatus.salmon_path(user)}" />
       <link rel="self" href="#{OStatus.feed_path(user)}" />
       <author>
         #{user_xml}