Provide basic infrastructure for moving PHIDs, Handles and Object Names to applications

Summary:
See discussion in T2715. Currently, PHIDs are all hard coded in the PHID application. In the long run, we need to move them out into actual applications.

A specific immediate issue is Releeph, which uses a very very old and very broken mechanism to inject PHIDs in a way that only sort of works.

Moving forward, every PHID type will be provided by a `PhabricatorPHIDType` subclass, which will manage loading it, etc.

This also moves toward cleaning up the "load objects by name" (where "name" means something like `D12`) code, which is an //enormous// mess and spread across at least 4-5 callsites.

Test Plan: Used `phid.lookup` and `phid.query` to load Slowvotes.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6502