Added a greeting to the changeset comment message

The greeting includes the recipient's user name (as for the message notification mail), which should increase the recipient's trust that the message isn't spam. Greetings were reused from each language's respective locale flie.

app/views/notifier/changeset_comment_notification.html.erb

@@ -1,5 +1,9 @@
 <% content_for :body do %>
   <p style="margin: 0">
+    <%= t'notifier.changeset_comment_notification.hi', :to_user => @to_user %>
+  </p>
+
+  <p>
     <% if @owner %>
       <%= raw t "notifier.changeset_comment_notification.commented.your_changeset", :commenter => link_to_user(@commenter), :time => @time %>
     <% else %>