Add images to individual posts for social sharing

[Eyas]

I updated our template such that:

• Jekyll variable page.image adds an image to the page itself and appears as its social sharing image.
• Jekyll variable page.sharing_image sets the image to the social sharing image only (if page.image is not set)
• Jekyll variable site.sharing_image is used for social sharing otherwise

We want to go over our pages and decide which ones could use a more specific image or photo associated with it. We will need to make sure we are using images that have reusable licenses or are in the public domain.

To add an image, first, upload it to the img/ directory, then set it on a particular issue .md file in the top section for variables.

[Eyas]

cc @stefmcneely74