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/helpers/pool_versions_helper.rb

@@ -1,29 +1,38 @@
 module PoolVersionsHelper
-  def pool_version_show_diff(pool_version)
-    previous = pool_version.previous
-    previous.present? && pool_version.description != previous.description
+  def pool_version_show_diff(pool_version, type)
+    other = pool_version.send(type)
+    other.present? && pool_version.description != other.description
   end
 
-  def pool_version_name_diff(pool_version)
-    previous = pool_version.previous
-    if previous.present? && (pool_version.name != previous.name)
-      name_diff = diff_name_html(pool_version.pretty_name, previous.pretty_name)
+  def pool_version_name_diff(pool_version, type)
+    other = pool_version.send(type)
+    if other.present? && (pool_version.name != other.name)
+      if type == "previous"
+        name_diff = diff_name_html(pool_version.name, other.name)
+      else
+        name_diff = diff_name_html(other.name, pool_version.name)
+      end
       %(<br><br><b>Rename:</b><br>&ensp;#{name_diff}</p>).html_safe
     else
       ""
     end
   end
 
-  def pool_version_post_diff(pool_version)
-    previous = pool_version.previous
+  def pool_version_post_diff(pool_version, type)
+    other = pool_version.send(type)
     diff = {}
 
-    if previous.present?
-      diff[:added_post_ids] = pool_version.post_ids - previous.post_ids
-      diff[:removed_post_ids] = previous.post_ids - pool_version.post_ids
-    else
+    if other.present? && type == "previous"
+      diff[:added_post_ids] = pool_version.post_ids - other.post_ids
+      diff[:removed_post_ids] = other.post_ids - pool_version.post_ids
+    elsif other.present?
+      diff[:added_post_ids] = other.post_ids - pool_version.post_ids
+      diff[:removed_post_ids] = pool_version.post_ids - other.post_ids
+    elsif type == "previous"
       diff[:added_post_ids] = pool_version.added_post_ids
       diff[:removed_post_ids] = pool_version.removed_post_ids
+    else
+      return ""
     end
 
     render "pool_versions/diff", diff: diff