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 that I just needed for technical reasons and/or to pass lint. The next couple patches will move logic into them and use ApplicationSearch properly.

Test Plan:
  - Searched for stuff.
  - Searched for stuff with filters.
  - Searched for fulltext in Maniphest.
  - Grepped for `PhabricatorSearchQuery`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4365

Differential Revision: https://secure.phabricator.com/D8120