This PR fulfills https://ctds-planx.atlassian.net/browse/PXP-4733

Let filters in data explorer support nested fields

To add a nested field to filter, just denote it with a .
Example:

  "filters": {
    "tabs": [
      {
        "title": "Subject",
        "fields": [
          "project_id",               --> a regular field
          "visits.visit_label"      --> a nested field
        ]
      }
    ]
  },

By default any . and/or _ will be converted to whitespaces and the character immediately after that will be captialized, e.g.: visits.visit_label will be displayed as Visits Visit Label.
To overwrite, use fieldMapping,
Example:

  "guppyConfig": {
    "dataType": "subject",
    "fieldMapping": [
      {"field": "visits.days_to_visit", "name": "Nested Days to Visit"}
    ]
  }

Deployed in https://qa-mickey.planx-pla.net/explorer

New Features

• Let filters in data explorer support nested fields

Improvements

• Selected filters will always show before unselected ones in filter sections