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/versions/_types.html.erb

@@ -0,0 +1,6 @@
+<div id="version-comparisons">
+  <span>Version comparison:</span>
+  <ul id="version-comparisons-list">
+    <%= version_type_links(params) %>
+  </ul>
+</div>