R1:a48128d36fc6

Partially use ApplicationSearch in main search

Summary:
Ref T4365. Primary search currently uses `PhabricatorSearchQuery` for storage, which is pretty much the same as `PhabricatorSavedQuery`, except that it's old and not used anywhere else anymore.

Maniphest used to also use this table, but no longer does after Septmeber, 2013. We need to retain the class so the migration can work.

This introduces `PhabricatorSearchApplicationSearchEngine` and `PhabricatorSearchDocumentQuery`, but they're both stubs…