Remove admin requirement from MetaMTASendGridReceiveController

Summary:
This got caught in the crossfire when we admin-only'd the whole MetaMTA tool. It
should not be admin only.

(Generally, we should probably separate this out better at some point.)

Test Plan: Hit /mail/sendgrid/ as a logged-out, non-admin user (like SendGrid
does).

Reviewers: s, btrahan

Reviewed By: s

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1588