Make edge types modular

Summary:
Ref T5245. I want to add a new capability to edge types, which is a good opportunity to move away from PhabricatorEdgeConfig, which isn't modular.

This is basically the same as the modularization of PHID types, which has worked well. Add PhabricatorEdgeType and provide an adaption layer for the existing code.

This has no runtime changes, except the fixed edge constant.

Test Plan: Ran var_dump(PhabricatorEdgeType::getAllTypes()) and got reasonable looking output.

Reviewers: chad, btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9837