Add capability for social preview images and text

Currently nothing is generated on social preview for donate.torproject.org. We will need to add the capability to add metatags and metacards so that there is a preview that generates on social sites, example here: https://socialsharepreview.com/?url=https://torproject.org

We will need to add a custom graphic once the year end campaign launches - I will add this note to the appropriate ticket or create a new one so it doesn't get lost.

For now, can we use the same graphic that is generated on the landing page for torproject.org?

Here are the metatags that we should have for now. I pulled these from a waybackmachine url on an SEO tool from donate-paleo, but these should be accurate:

• title Tor Project | Donate
• description Stand up for the right to privacy. Donate to the Tor Project.
• viewport width=device-width, initial-scale=1
• twitter:card summary_large_image
• twitter:site @torproject
• twitter:creator @torproject
• twitter: image (graphic used on torproject.org)

Thank you @lavamind! <3