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/javascript/src/styles/specific/wiki_page_versions.scss

@@ -9,10 +9,6 @@ div#c-wiki-page-versions {
       background: var(--wiki-page-versions-diff-ins-background);
       text-decoration: none;
     }
-
-    ul.wiki-other-names-diff-list li.obsolete {
-      text-decoration: dotted underline;
-    }
   }
 
   #a-index {