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/artist_versions/_listing.html.erb

@@ -4,17 +4,17 @@
       <% t.column "Name", td: {class: "diff-body"} do |artist_version| %>
         <%= link_to artist_version.name, artist_path(artist_version.artist_id) %>
         <%= link_to "»", artist_versions_path(search: {artist_id: artist_version.artist_id}, anchor: "artist-version-#{artist_version.id}") %>
-        <%= artist_version_name_diff(artist_version) %>
+        <%= artist_version_name_diff(artist_version, params[:type]) %>
       <% end %>
       <% t.column "Other Names", td: {class: "diff-body"} do |artist_version| %>
-        <%= artist_version_group_name_diff(artist_version) %>
-        <%= artist_version_other_names_diff(artist_version) %>
+        <%= artist_version_group_name_diff(artist_version, params[:type]) %>
+        <%= artist_version_other_names_diff(artist_version, params[:type]) %>
       <% end %>
       <% t.column "URLs", td: {class: "col-expand"} do |artist_version| %>
-        <%= artist_version_urls_diff(artist_version) %>
+        <%= artist_version_urls_diff(artist_version, params[:type]) %>
       <% end %>
       <% t.column "Changes" do |artist_version| %>
-        <%= status_diff_html(artist_version) %>
+        <%= status_diff_html(artist_version, params[:type]) %>
       <% end %>
       <% t.column "Updated" do |artist_version| %>
         <%= link_to_user artist_version.updater %>

app/views/artist_versions/index.html.erb

@@ -2,6 +2,8 @@
   <div id="a-index">
     <h1>Artist History</h1>
 
+    <%= render "versions/types" %>
+
     <%= render "listing" %>
 
     <%= numbered_paginator(@artist_versions, :search_count => params[:search]) %>