Highlight document contents with hit number attribute

[jan-niestadt]

(see https://github.com/INL/corpus-frontend/issues/527)

/docs/DOCID/contents should add a hit number to <hl> tags, which will allow the frontend to deal with overlapping hits and hits that e.g. span across <s> tags.

<s><hl n="1">The fox <hl n="2">jumps</hl></hl><hl n="2"> over the dog.</hl></s>