Pipeline email notifications have no "To:" header
The email notifications sent out for pipeline success/failure don't include a `To: xxx` header and this causes some odd behavior in certain email clients.
### Apple Mail (iOS 10):
#### Headers sent:
```
Delivered-To: mgreiling@gitlab.com
Received: by 10.202.182.135 with SMTP id g129csp210989oif;
Mon, 21 Nov 2016 09:34:59 -0800 (PST)
X-Received: by 10.36.196.86 with SMTP id v83mr11532999itf.86.1479749699677;
Mon, 21 Nov 2016 09:34:59 -0800 (PST)
Return-Path: <bounce+a1e52f.947b4-mgreiling=gitlab.com@mg.gitlab.com>
Received: from so254-240.mg.gitlab.com (so254-240.mg.gitlab.com. [198.61.254.240])
by mx.google.com with ESMTPS id 11si15043980iob.187.2016.11.21.09.34.59
for <mgreiling@gitlab.com>
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Mon, 21 Nov 2016 09:34:59 -0800 (PST)
Received-SPF: pass (google.com: domain of bounce+a1e52f.947b4-mgreiling=gitlab.com@mg.gitlab.com designates 198.61.254.240 as permitted sender) client-ip=198.61.254.240;
Authentication-Results: mx.google.com;
dkim=pass header.i=@mg.gitlab.com;
spf=pass (google.com: domain of bounce+a1e52f.947b4-mgreiling=gitlab.com@mg.gitlab.com designates 198.61.254.240 as permitted sender) smtp.mailfrom=bounce+a1e52f.947b4-mgreiling=gitlab.com@mg.gitlab.com;
dmarc=pass (p=REJECT dis=NONE) header.from=gitlab.com
DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mg.gitlab.com; q=dns/txt;
s=mailo; t=1479749699; h=Content-Transfer-Encoding: Content-Type:
Mime-Version: Subject: Message-ID: Reply-To: From: Date: Sender;
bh=COIfOQJcVwkutLkKpII5HLd+DF6dtcqr1+pMM+8zq6c=; b=HDOBRH94xn8WbK7Df35kx92z9mW7ba7Px45/hlQkZK67e9IDyqeMMowybD7zpxMF9Mu9SHgl
GDCJug84jUDJrAMErXU5w5xCOpB30q/867Ro1ZcfG5K2M677sENezr+PHaI9HIGg2xXSvXBt
nILxx0FO10CCNM1jUUQvgxizUbk=
DomainKey-Signature: a=rsa-sha1; c=nofws; d=mg.gitlab.com; s=mailo;
q=dns; h=Sender: Date: From: Reply-To: Message-ID: Subject:
Mime-Version: Content-Type: Content-Transfer-Encoding;
b=ZnymgYGoROrtABlvmYZUwGYQhv/qFfldTSqENwWgx/Yh8NdVHgq84KKgb4LGvtfYyLKaGS
cbGAZOlNceq4E70OXtCBgTKwIRXRipZzomejaaypmKDO09X+uKzw4DEdxJCLktnAf7kinjuY
6UMwO+PgGG7Vr8iFx8O9YIanmxocs=
Sender: gitlab@mg.gitlab.com
X-Mailgun-Sending-Ip: 198.61.254.240
X-Mailgun-Sid: WyIxYzE5MiIsICJtZ3JlaWxpbmdAZ2l0bGFiLmNvbSIsICI5NDdiNCJd
Received: from mg.gitlab.com (Unknown [104.208.241.47])
by mxa.mailgun.org with ESMTP id 58333043.7fbfd869c0d8-smtp-out-n03;
Mon, 21 Nov 2016 17:34:59 -0000 (UTC)
Date: Mon, 21 Nov 2016 17:34:58 +0000
From: GitLab <gitlab@mg.gitlab.com>
Reply-To: GitLab <noreply@gitlab.com>
Message-ID: <58333042b2d4f_d7733fc7122152001091c@worker3.cluster.gitlab.com.mail>
Subject: GitLab Community Edition | Pipeline #5076458 has succeeded for
24499-fix-activity-autoload-on-large-viewports | 0dd95ca9 in !7608
Mime-Version: 1.0
Content-Type: multipart/alternative;
boundary="--==_mimepart_58333042b213f_d7733fc71221520010903b";
charset=UTF-8
Content-Transfer-Encoding: 7bit
X-GitLab-Project: GitLab Community Edition
X-GitLab-Project-Id: 13083
X-GitLab-Project-Path: gitlab-org/gitlab-ce
X-GitLab-Pipeline-Id: 5076458
X-GitLab-Pipeline-Ref: 24499-fix-activity-autoload-on-large-viewports
X-GitLab-Pipeline-Status: success
----==_mimepart_58333042b213f_d7733fc71221520010903b
Content-Type: text/plain;
...
...
...
```
/cc @godfat
issue