Improvements to OpenGraph/TwitterCard

[obilodeau]

• handle when the featured image is actually a video, like here: https://montrehack.ca/2021/04/21/canada-crackme.html
• handle when the featured image is hosted externally
• verify archives for failure cases including before we were using the featured_img yaml front-matter
• add a summary tag everywhere too

Validators:

• https://cards-dev.twitter.com/validator
• https://developers.facebook.com/tools/debug/