Add Custom Settings to Apps Endpoint of Portal API

Adds the custom settings to the object that is returned when calling the apps endpoint of the Tethys Portal REST API. This makes the custom settings available to apps with the React frontend, which uses the REST API to get info about the app.

The custom settings are only added if the request user is authenticated. Otherwise, the endpoint returns only the basic metadata about the app.
This can include all types of Custom Settings: JSON, Secret, and Simple.
Which custom settings are included is controlled with an new field on the CustomSettingsBase model: include_in_api. The setting is included in the API response if include_in_api is True. Defaults to False.

Example of object returned with custom settings:

{
  "title": "tRIBS Model Builder",
  "description": "An application for building new tRIBS models for SRP studies.",
  "tags": "",
  "package": "tribs",
  "urlNamespace": "tribs",
  "color": "#115590",
  "icon": "/static/tribs/images/icon.gif",
  "exitUrl": "/apps/",
  "rootUrl": "/apps/tribs/home/home/",
  "settingsUrl": "/admin/tethys_apps/tethysapp/3/change/",
  "customSettings": {
    "cesium_api_key": {
      "type": "STRING",
      "value": "some-api-key"
    },
    "json_setting": {
      "type": "JSON",
      "value": {"foo": "bar"}
    },
    "secret_setting": {
      "type": "SECRET",
      "value": "some-secret"
    }
  }
}

Also in this PR:

Unpinned sphinx version - seems to be working again
Updated configuration for newer version of sphinx theme

tethysplatform / tethys

Add Custom Settings to Apps Endpoint of Portal API #1004