RSS: Make sure post URL is the first <link> element

Otherwise some RSS readers (tested in Miniflux) might pick the context
URL as the external link.

Related to #2425.

lib/pleroma/web/templates/feed/feed/_activity.rss.eex

@@ -9,7 +9,6 @@
   <ostatus:conversation ref="<%= activity_context(@activity) %>">
     <%= activity_context(@activity) %>
   </ostatus:conversation>
-  <link rel="ostatus:conversation"><%= activity_context(@activity) %></link>
 
   <%= if @data["summary"] do %>
     <description><%= escape(@data["summary"]) %></description>
@@ -21,6 +20,8 @@
     <link><%= @data["external_url"] %></link>
   <% end %>
 
+  <link rel="ostatus:conversation"><%= activity_context(@activity) %></link>
+
   <%= for tag <- @data["tag"] || [] do %>
     <category term="<%= tag %>"></category>
   <% end %>