Add API for getting facets.

[colinrobinsonuib]

It is already possible to search using facets with the existing search API.

Example:

/api.php?action=query&list=search&format=json&srsearch=john+haswbfacet%3AP1%3DQ91&srnamespace=120

Is the same as searching john haswbfacet:P1=Q91 on the search page. But this only gives you the results, not the facets available to do another search. Getting those facets is crucial for building a headless application.

You can search on the elasticsearch server directly as I show in this comment #275 (comment)

This PR adds a new API endpoint for retriving those values directly through mediawiki without needing to expose the elasticsearch server.

Example usage:

api.php?action=wbfacetsearch&search=john+haswbfacet:P1=Q91&format=json&namespaces=120

Response:

{
  "wbfacetsearch": [
    {
      "property": "P55",
      "label": "country of citizenship",
      "values": [
        {
          "value": "Q365",
          "count": 6,
          "label": "United States"
        },
        {
          "value": "Q372",
          "count": 2,
          "label": "Australia"
        },
        {
          "value": "Q366",
          "count": 1,
          "label": "United Kingdom"
        },
        {
          "value": "Q373",
          "count": 1,
          "label": "Netherlands"
        },
        {
          "value": "Q389",
          "count": 1,
          "label": "Republic of Ireland"
        },
        {
          "value": "Q93",
          "count": 1,
          "label": "Canada"
        }
      ]
    },
    {
      "property": "P56",
      "label": "date of birth",
      "values": [
        {
          "value": -5837961600000,
          "count": 1,
          "label": "-5837961600000"
        },
        {
          "value": -1262304000000,
          "count": 1,
          "label": "-1262304000000"
        },
        {
          "value": -441849600000,
          "count": 1,
          "label": "-441849600000"
        },
        {
          "value": -378691200000,
          "count": 1,
          "label": "-378691200000"
        },
        {
          "value": 504921600000,
          "count": 1,
          "label": "504921600000"
        }
      ]
    }
  ]
}