Fix metadata so shares on twitter / yammer / other places look nice

[andylolz]

#192 added metadata, but there are some issues with it that mean shares aren’t quite right.

You can debug the open graph metadata using the facebook linter. For example:
https://developers.facebook.com/tools/debug/sharing/?q=https%3A%2F%2Fiatistandard.org%2Fen%2Fnews%2Fsubmit-bids-for-iati-data-use-fund%2F

There are a couple of warnings shown. The protocol is missing from og:url, and the base URL is missing from the og:image.

[andylolz]

Similarly, the twitter card validator shows that the twitter:image needs to be an absolute URL.

[andylolz]

Fixed in #367.

[andylolz]

#367 has now been deployed (which is great) but it didn’t quite fix the problem.

Looks good on twitter:

…but the image used in this facebook example still doesn’t show, because it’s too small. Perhaps social_share_image_url should be a bit bigger?

Provided og:image URL, https://iatistandard.org/media/images/duf1.width-320.jpegquality-60.png was not valid because it did not meet the minimum size constraint of 200px by 200px.