Currently, the image is resized to be 600px, which will work for
most images but caused issues for images which have very wide
aspect ratios. This change updates the partials to use a Fill with
an appropriate bounding box, rather than a simple resize.
This PR changes this by grouping the links within an object key: `Site.Data.sharing.links`. This means that sites can provide their own
`data/sharing.json` file and overwrite the `links` key, to provide links of their own links.
Currently the images are displayed at their full resolution. Those images could be very large, but end up being displayed as 300px wide. This changes that by resizing those images to a more suitable size.
Chris Banes