Restored previous entity retrieval

Using temporary tables turned out to be more expensive on bigger databases.
Restoring two queries version.
Further tweaks to model

- Fixed broken negative searches
- Faster search by tag / comment / submit / favorites (useless nested joins
  replaced to entity prefetch). Side effect: searching for nonexistent tags,
  users etc yields informative errors instead of "no posts to show")
- Fixed duplicated column in order clause ("ORDER BY id DESC, id DESC")