Construct ES queries in backend

[lukavdplas]

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).

[BeritJanssen]

Places where the frontend now directly construct ES queries

• search queries
• download
• visualization (histograms construct the visualization query itself, others send search query)