Get query button working in explorev2 (#1581)

* Get query buttonw working in explorev2

 - Create new endpoint for updating explore viz
 - Send over new form_data when query button is pressed

* Added endpoint test

* Changes based on comments

* Added docstring for endpoint, and query spec

* Remove white space around docstring

superset/assets/javascripts/explorev2/stores/store.js

@@ -604,7 +604,7 @@ export const visTypes = {
     label: 'Heatmap',
     controlPanelSections: [
       {
-        label: null,
+        label: 'Axis & Metrics',
         fieldSetRows: [
           ['all_columns_x'],
           ['all_columns_y'],