Summary:
Ref T12819. Supports negating search terms, e.g. "apple -honeycrisp".

When negating a term, we're a little more strict about what can match (that is, what can //prevent// a document from being returned) since it's easy for a user to type "apple -honeycrisp -honey -crisp -crispies -olcrispers -honeyyums" to keep refining their search, but hard/impossible to split apart an overboard term.

Test Plan:
  - Ran `apple -smith`, `apple -"granny smith"`, etc.
  - Verified `phone -tact` does not exclude `phone contact`.
  - (In theory, `phone -~tact` would, but the parser currently doesn't support this, and I'm not champing at the bit to add support.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18502