highlightField View Helper: also apply substring highlights

The highlighter may return substrings only when the field content is long.
Do not fully replace the original string but replace the matched substring
instead.
germania-sacra: more aggressive highlighting

This is technically wrong but seems to give better results.
E.g. when searching for »Augustiner«, some results only have »Augustiner«
as orden but their title only contains »Augustinerchorherrenstift« which
does not match. It should not match that word but the result list looks
strange when it isn’t highlighted.