Construct ES queries in backend

I'm making this issue mostly as reference point, as the idea has come up a few times (e.g. #1292 , #1099 )

For the most part, elasticsearch queries are constructed entirely by the frontend and forwarded to elasticsearch by the backend. There are some exceptions to this:

- The backend analysis for visualisation often includes adjustments to the search query, such as adding extra filters.
- Since 5.3.0, a query to the backend includes an `es_query` and an optional key `tags` that specifies user tags to filter on.

It would be more consistent if we moved the conversion to and from an elasticsearch query _entirely_ to the backend and used an I-analyzer-specific "intermediary representation" in the API.

This makes it easier to implement certain features (such as tagging).