Search for search terms no further than *n* words apart

**Is your feature request related to a problem? Please describe.**
This request came as a potential future request from the People & Parliament team: they would like to search for two search terms and make sure that they occur within close proximity to each other, not just anywhere in a document

**Describe the solution you'd like**
It seems the [Elasticsearch intervals query](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-intervals-query.html) might fit the bill. So I think the technical implementation should not be too much of an issue, but how to reflect this different type of query of the UI requires some consideration.

**Describe alternatives you've considered**
We might also post-process documents which are matches to the simple query string query - but I'm not sure that would be a better solution, as the UI question remains.

**Additional context**
Add any other context or screenshots about the feature request here.