Contact form email template
If you check core.services.yml you can see the service which sends mail (plugin.manager.mail).  It looks for  overrides for default mail plugin starting from message_id to module name. You can create simple new mail plugin which sends HTML mail and set this in configuration "system.mail".

 drush cset system.mail interface.contact your_mail_plugin  // this will change mail plugin for contact module


Creating a mail plugin is easy and you can find example here Drupal\Core\Mail\Plugin\Mail

MailSystem  module does this and it supports advanced options https://www.drupal.org/project/mailsystem