Highlight the search terms in the results

[jgonggrijp]

Either using Alto (but this only makes sense if we also show the scanned image, which we don't do yet) or by applying a quick client-side search on the plain text (as suggested by @oktaal).

[oktaal]

@JosedeKruif mentioned that similar functionality was already built for https://github.com/UUDigitalHumanitieslab/LoCMiner. So I'll look into that first.

[oktaal]

The highlighting is added by the LoC API, so this cannot be reused for this.