Add meta tags to cause CFP pages to have a sensible image, heading, and snippet when shared on social media

[rolandturner]

At present when a link to a CFP (e.g. https://eventyay.com/e/55d2a466/cfs ) is shared on LinkedIn, it looks like this:

It would look better if the og:image, twitter:image, etc. tags were present in the page header so that the presentation in social media looked sensible.

It's worth testing in at least:

• Facebook
• LinkedIn
• Twitter (although they've made noises about withdrawing the capability...)
• Mastodon
• Any other social media channels that FOSSASIA is already promoting itself through