Migrate Maniphest task blockers to modern EdgeType classes
Prevents "edited tasks, added: 1; removed: 1"
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