/wiki_page_versions: various usability improvements.

Changes to the /wiki_page_versions global listing:

* Add "diff" links that show you what changed in the given edit.
* Add "?" links that take you to the current version of the wiki.
* Add "»" links next to wiki page titles that take you to the wiki's full edit history.
* Add "»" links next to usernames that take you to the user's full edit history.
* Add a "Status" column that shows whether the wiki page was created,
  deleted, undeleted, or renamed.
* Link to /wiki_page_versions in sidebar, not /wiki_pages?order=time.

app/models/wiki_page_version.rb

@@ -5,8 +5,6 @@ class WikiPageVersion < ApplicationRecord
   belongs_to :artist, optional: true
   delegate :visible?, :to => :wiki_page
 
-  extend Memoist
-
   module SearchMethods
     def for_user(user_id)
       where("updater_id = ?", user_id)
@@ -41,7 +39,6 @@ class WikiPageVersion < ApplicationRecord
   def previous
     WikiPageVersion.where("wiki_page_id = ? and id < ?", wiki_page_id, id).order("id desc").first
   end
-  memoize :previous
 
   def category_name
     Tag.category_for(title)