Added DeckGL.Polygon Layer w/ JS controls (#4227)

* Working polygon layer for deckGL

* add js controls

* add thumbnail

* better description

* refactor to leverage line_column controls

* templates: open code and documentation on a new tab (#4217)

As they are external resources.

* Fix tutorial doesn't match the current interface #4138 (#4215)

* [bugfix] markup and iframe viz raise 'Empty query' (#4225)

closes https://github.com/apache/incubator-superset/issues/4222

Related to: https://github.com/apache/incubator-superset/pull/4016

* [bugfix] time_pivot entry got missing in merge conflict (#4221)

PR here https://github.com/apache/incubator-superset/pull/3518 missed a
line of code while merging conflicts with time_pivot viz

* Improve deck.gl GeoJSON visualization (#4220)

* Improve geoJSON

* Addressing comments

* lint

* refactor to leverage line_column controls

* refactor to use DeckPathViz

* oops

superset/assets/visualizations/deckgl/layers/polygon.jsx

@@ -0,0 +1,28 @@
+import { PolygonLayer } from 'deck.gl';
+
+import * as common from './common';
+import sandboxedEval from '../../../javascripts/modules/sandbox';
+
+export default function polygonLayer(formData, payload, slice) {
+  const fd = formData;
+  const fc = fd.fill_color_picker;
+  let data = payload.data.features.map(d => ({
+    ...d,
+    fillColor: [fc.r, fc.g, fc.b, 255 * fc.a],
+  }));
+
+  if (fd.js_datapoint_mutator) {
+    // Applying user defined data mutator if defined
+    const jsFnMutator = sandboxedEval(fd.js_datapoint_mutator);
+    data = data.map(jsFnMutator);
+  }
+
+  return new PolygonLayer({
+    id: `path-layer-${fd.slice_id}`,
+    data,
+    filled: fd.filled,
+    stroked: fd.stoked,
+    extruded: fd.extruded,
+    ...common.commonLayerProps(fd, slice),
+  });
+}