Consolidate search-result.html.twig and search-results.html.twig

The search result templates split appart the OL and the LI parts of an item list. This goes against the principal of keeping single HTML elements in single template files, one of our principles for cleaning up the theme layer.

I propose that we consolidate these two template files into one, and use twig to do the looping within search-results.html.twig.

Related:
#1898448: Convert search module to Twig