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/application/_diff_list.html.erb

@@ -2,11 +2,11 @@
 
 <%= tag.ul class: [*ul_class] do %>
   <% diff.added.each do |item| %>
-    <%= tag.li item, class: ["added", ("obsolete" if item.in?(diff.obsolete_added)), *li_class] %>
+    <%= tag.li item, class: ["added", *li_class] %>
   <% end %>
 
   <% diff.removed.each do |item| %>
-    <%= tag.li item, class: ["removed", ("obsolete" if item.in?(diff.obsolete_removed)), *li_class] %>
+    <%= tag.li item, class: ["removed", *li_class] %>
   <% end %>
 
   <% diff.changed.each do |old, new| %>