Modularize global quick create builtin items

Summary: Ref T5867. Instead of hard-coding projects, tasks and repositories, let EditEngines say "I want a quick create item" so third-party code can also hook into the menu without upstream changes.

Test Plan: Saw same default items in menu.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5867

Differential Revision: https://secure.phabricator.com/D17215