Adds Composer 2 support

Symfony 4.3 added a new Mailer component with support for local SMTP servers, third-party mailing services (Amazon, Mailgun, Google, etc.) and the option to send emails asynchronously.

Problem/Motivation Follow-up from [#1856562-31]. 1) An attacker can sent contact messages with arbitrary unfiltered HTML. This is a concern for security however it has been cleared by the security team for discussion in public, see #59. To reproduce: 1. Enable the contact module 2. Enable the swiftmailer module to allow sending HTML mails 3. Use the mailsystem module to set swiftmailer as the default formatter and transport 4. Grant permission for anonymous users to use the site-wide contact form