Add an "only()" edge logic constraint, meaning "only the other constraints, exactly"

Summary:
See PHI57. For example, a query for "ios, only()" finds tags tasked with iOS, exactly, and no other tags.

I called this "only()" instead of "exact()" because we use the term/function "Exact" elsewhere with a different meaning, e.g. in Differential.

Test Plan:
Basic query for a tag:

Same query with "only", finds tasks tagged with only that tag:

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18543