Add support for significant text aggregation

[lukasolson]

Elasticsearch has a significant text aggregation (added in 6.0 I believe). We should add support for it inside all of the corresponding visualizations.

[elasticmachine]

Pinging @elastic/kibana-app

[danielyahn]

+1 it would be really nice to have for tag cloud viz

[gianluigiB]

+1 I'd like to use it in a data table or tag cloud together with a search in a dashboard.

When user selects one of the results in either visualizations, he/she would see where they occur within the analyzed text field of resulting documents displayed by the search.
This has huge value when trying root-cause analysis of issues described in the analyzed text field and characterized by the significant text aggregation.

Doing the same with significant terms aggregation on analyzed text (using also n-grams) quickly fills up fielddata, making that way unpractical.

[timroes]

Same comment from here applies, since we need to ignore the non-aggregatable flag for those fields, and actually be able to filter based on the original ES type not our Kibana field type.

[markharwood]

Another example of a user jumping through hoops to turn text into keyword fields because the tag cloud doesn't support significant_text.