Bring PHPMailer config into the new config UI

Summary:
Port PHPMailer options. Also:

• Don't show values on config lists if they're masked (this is mostly for passwords, to prevent them from being idly/accidentally disclosed).
• Don't show "default" icon -- just show an icon if the value has been customized. This makes it easier to pick out custom values.

Test Plan: Looked at / edited mailer values.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4441