Migrate Maniphest task blockers to modern EdgeType classes

Summary:
Prevents "edited tasks, added: 1; removed: 1"

Fixes T6757, using D9839 as an example

Test Plan: Added and removed blockers to/from tasks, saw the expected history entries.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6757

Differential Revision: https://secure.phabricator.com/D11045