Add support for configuring highlights and collection images

[anjackson]

For the new website, we need a way to configure which collections are to be highlighted on the homepage, and to provide a suitable image for a collection.

This can then be used to populate the Collection & Targets Solr Instance, and then the ukwa-ui can be modified to make use of the data.

Open Question: Where to store the images?

[anjackson]

I think the work in #578 will follow on nicely from this, assuming we proceed as I've outlined there (by extending the Edit Collections page).