search: refactor PostQueryBuilder class methods into instance methods.

* Make scan_query, parse_query, normalize_query into instance methods
  instead of class methods. This is to a) clean up the API and b)
  prepare for moving certain tag utility methods into PostQueryBuilder.

* Fix a few cases where a caller used scan_query when they should have
  used split_query or parse_tag_edit.

app/logical/alias_and_implication_importer.rb

@@ -88,8 +88,8 @@ class AliasAndImplicationImporter
         all
 
       when :mass_update
-        all += PostQueryBuilder.split_query(token[1])
-        all += PostQueryBuilder.split_query(token[2])
+        all += PostQueryBuilder.new(token[1]).split_query
+        all += PostQueryBuilder.new(token[2]).parse_tag_edit
         all
 
       when :change_category