Remove subscribe icons
Summary: Used more logical icons for subscribe, auto, and delete instead of the mail icons. Fixes T3329
Test Plan: Tested subscribing and unsubscribing in Maniphest.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T3329
Differential Revision: https://secure.phabricator.com/D6151