authorize.php should be used for all Installer module downloads/installs

Now that we have fixed the usage of authorize.php when using Installer module for manual module installs (#1920), it's become clear that we should be using authorize.php for all downloading/installing of modules/themes/layouts.

Without using authorize.php, Installer module only works if the owner of web root directory and the web server user are the same. By using authorize.php, we could enable users to more securely set up their site and still allow modules to be downloaded/installed. If the owner of the web root is the web server user (as is the case in most shared hosting environments and Pantheon), then we don't have a problem. But when these users are different, authorize.php could enable the use of the Installer UI.