Convert Mailer module from PHPMailer to Swiftmailer

Convert Mailer module from PHPMailer to Swiftmailer
fixes #1702 and refs #1658

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1702
| Refs tickets  | #1658
| License       | MIT
| Doc PR        | -