Allow site mail and account mail to be set separately

[kmbremner]

Is your feature request related to a problem? Please describe.
Currently BLT uses the same value (drupal.account.mail) for the site-mail and account-mail values:

    /** @var \Acquia\Blt\Robo\Tasks\DrushTask $task */
    $task = $this->taskDrush()
      ->drush("site-install")
      ->arg($this->getConfigValue('project.profile.name'))
      ->rawArg($this->getConfigValue('setup.install-args'))
      ->option('sites-subdir', $this->getConfigValue('site'))
      ->option('site-name', $this->getConfigValue('project.human_name'))
      ->option('site-mail', $this->getConfigValue('drupal.account.mail'))
      ->option('account-name', $username, '=')
      ->option('account-mail', $this->getConfigValue('drupal.account.mail'))
      ->option('locale', $this->getConfigValue('drupal.locale'))
      ->verbose(TRUE)
      ->printOutput(TRUE);

We don't always want the site email to be the same as the admin user's email.

Describe the solution you'd like
Make it possible to specify separate values for site-mail and account-mail.

Describe alternatives you've considered
It's easy to manually change values after site install and capture in config.

Additional context
Add any other context or screenshots about the feature request here.

[danepowell]

Sounds like a fine feature as long as it's backwards compatible (the new config parameter should default to the value of drupal.account.mail). Should be a pretty straightforward PR if you're interested in contributing.