Add highlighting inside Discover

When searching inside discover, we want to highlight in the search results where the query matched.

Elasticsearch has an option you can set in the request, `highlight`. In the response, a new field, `highlight`, will have fragments corresponding to where in the field the query matched. There can be multiple `highlights` per matched field, and multiple fields could match.

The results from `highlight` should be shown inside both the discover row, as well as the expanded details, as well as in `_source` inside the table row (if it is a selected field). Inside `_source`, we'd like to show fields that matched the query before all other fields.

Closes #1644.