Add alternate comparison types to versions

- The types are:
-- Previous: The default and the previously used type
-- Subsequent: Compares against the next version
-- Current: Compares against the current version
- Allow switching between comparison types in index and diff views
-- Have links vary depending upon current comparison type

app/views/note_versions/index.html.erb

@@ -2,6 +2,8 @@
   <div id="a-index">
     <h1>Note History</h1>
 
+    <%= render "versions/types" %>
+
     <%= render "listing" %>
 
     <%= numbered_paginator(@note_versions) %>