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/note_versions_helper.rb

@@ -1,23 +1,36 @@
 module NoteVersionsHelper
-  def note_version_position_diff(note_version)
-    previous = note_version.previous
+  def note_version_position_diff(note_version, type)
+    other = note_version.send(type)
 
     html = "#{note_version.x},#{note_version.y}"
-    if previous.nil? || (note_version.x == previous.x && note_version.y == previous.y)
+    if other.nil? || (note_version.x == other.x && note_version.y == other.y)
       html
+    elsif type == "previous"
+      "#{other.x},#{other.y} -> " + html
     else
-      "#{previous.x},#{previous.y} -> " + html
+      html + " -> #{other.x},#{other.y}"
     end
   end
 
-  def note_version_size_diff(note_version)
-    previous = note_version.previous
+  def note_version_size_diff(note_version, type)
+    other = note_version.send(type)
 
     html = "#{note_version.width}x#{note_version.height}"
-    if previous.nil? || (note_version.width == previous.width && note_version.height == previous.height)
+    if other.nil? || (note_version.width == other.width && note_version.height == other.height)
       html
+    elsif type == "previous"
+      "#{other.width}x#{other.height} -> " + html
     else
-      "#{previous.width}x#{previous.height}  -> " + html
+      html + " -> #{other.width}x#{other.height}"
+    end
+  end
+
+  def note_version_body_diff(note_version, type)
+    other = note_version.send(params[:type])
+    if type == "previous"
+      diff_body_html(note_version, other, :body)
+    else
+      diff_body_html(other, note_version, :body)
     end
   end
 end