Refactored search view to be API based and implemented complex filters.

This also includes other fixes:

- added django-dbgettext as a simple way to handle dynamic translations
  via the standard gettext API

- a fix for calculate_related_documents the cronjob function to
  actually use the correct table for document tags
Merge pull request #1583 from jezdez/extend-topic-filter-915760

Search refactor with translatable, admin configurable, complex search filters.