New ReactOS Website: Redo the Download page

Redo the Download page:

• Add a {< reactos-version >}

  shortcode and use it everywhere where we reference the latest ReactOS version (e.g. download URLs)

• Factor out the PayPal donation block from the donation page into a shortcode to reuse it for both the Download and Donation pages.
• Reimplement a window that asks for a donation when clicking the download link. Make it less annoying though, it should be easy to find the "No thanks" link (aka without scrolling down or sending a support mail).
• Possibly take inspiration from the current download pages of popular operating system projects.