/post_versions: add search form, tweak layout.

* Add search form above table.
* Move thumbnail to left of table when viewing history of single post.
* Remove unrelated links from subnav menu.
* Fix bugs with changed_tags search.

app/javascript/src/styles/specific/post_versions.scss

@@ -6,4 +6,13 @@ body.c-post-versions.a-index {
   .post-version-select-column {
     min-width: 2em;
   }
+
+  .advanced-search-link {
+    margin: 0 1em;
+  }
+
+  #p-revert-listing {
+    display: flex;
+    table#post-versions-table { flex: 1; }
+  }
 }