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