Show thumbnail of scan or image on search results overview

This requires a call to `request-media` from the `search-results.component`. This may be ultimately cleaner, as it also enables a finer-grained `*ngIf` on whether or not to display the button linking to the scans.

While originally this was implemented for Times and should work for the Times and Periodicals corpus, other corpora relying on pdfs (AnnualReports, Ecco, Guardian) won't be able to display thumbnails with an `` tag. This might be left for a later issue.