Sort global fulltext results by overall relevance

Summary:
Ref T12819. Currently, under the Ferret engine, we query each application's index separately and then aggregate the results.

At the moment, results are aggregated by type first, then by actual rank. For example, all the revisions appear first, then all the tasks.

Instead, surface the internal ranking data from the underlying query and sort by it.

Test Plan: Searched for "A B" with a task named "A B" and a revision named "A". Saw task first. Broadly, saw mixed task and revision order in result sets.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18551