R1:5243b0d65323

Move computeMailHash() to PhabricatorObjectMailReceiver

Summary: Kick this out of here. Ref T1205.

Test Plan: Grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1205

Differential Revision: https://secure.phabricator.com/D5942