[PR] Introduce "extended" site permissions to handle multiple levels of site options

By default we restrict sites to a specific upload filetype list and a 50MB filesize limit. In some cases we'll want to extend that, but only when deemed necessary by a global admin.

This provides an option in `site-info.php` that allows a global admin to assign extended permissions to a site. In doing so, the site can upload `.dmg` and `.zip` files, and the upload filesize limit is 100MB.

Global admins now have the ability to upload `.exe` files. Over time, once we get automated virus scanning going on the server, we can look into allowing that for more.