SQL overhaul: introducing tree-like queries

Reason: until now, PostSearchService was using magic to get around the biggest
limitation of SqlQuery.php: it didn't support arbitrary order of operations.
You couldn't join with something and tell then to select something from it.
Additionally, forging UPDATE queries was a joke. The new Sql* classes replace
SqlQuery completely and address these issues. Using Sql* classes might be
tedious and ugly at times, but it is necessary step to improve model layer
maintainability.

It is by no menas complete implementation of SQL grammar, but for current needs
it's enough, and, what's most important, it is easily extensible.

Additional changes:
* Added sorting style aliases
  - fav_count
  - tag_count
  - comment_count
* Sorting by multiple tokens in post search is now possible
* Searching for disliked posts with "special:disliked" always yields results
  (even if user has disabled showing disliked posts by default)
* More maintainable next/prev post support
Fixed HTML validation