Move more logic to the helper modules

- Diff view changes
-- Only show pool description changes in diff view
-- Conditionally render diff link when applicable values are changed
-- Conditionally show diff view sections when values are changed
- Show renames on index view
-- There is plenty of space
-- This wasn't shown at all for wikis
-- Having to navigate to an alternate page is unwieldy for pools
- Show "posts" as a status on pools
-- This is so all changes among versions are quantified as a status
- Standardize diff/index titles

app/views/post_versions/index.html.erb

@@ -1,9 +1,9 @@
 <div id="c-post-versions">
   <div id="a-index">
     <% if listing_type(:post_id) == :revert && @post_versions.present? %>
-      <h1>Tag History: <%= link_to "Post ##{params.dig(:search, :post_id)}", @post_versions[0].post %></h1>
+      <h1>Post History: <%= link_to "Post ##{params.dig(:search, :post_id)}", @post_versions[0].post %></h1>
     <% else %>
-      <h1>Tag History</h1>
+      <h1>Post History</h1>
     <% end %>
 
     <%= search_form_for(post_versions_path, classes: "inline-form") do |f| %>