moved bootstrap.php to src/
use Twig to render email templates

Now that we don't have to use a JSONView inside the API, to road is free to use
Twig to render the email templates. Still need to enable Twig in the API, though.
let the mailer render the templates, because it already takes care of "global" placeholders like 'domain' et al.
removed wonky vksprintf(), because we now use Twig to render such things

Also, vksprintf() is basically only used once throughout the entire code base.
For an email body. Which needs to be turned into a proper template.
we don't need these autoloading helpers anymore
moved add_editor_nav() into the charts controller
$recursive in mkdir() has been possible sind PHP 5.0
moved copy_recursively() into the plugin class