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/components/SelectField.jsx

@@ -7,7 +7,7 @@ import Select, { Creatable } from 'react-select';
 const propTypes = {
   name: PropTypes.string.isRequired,
   choices: PropTypes.array,
-  value: PropTypes.oneOfType([PropTypes.string, PropTypes.array]),
+  value: PropTypes.oneOfType([PropTypes.string, PropTypes.number, PropTypes.array]).isRequired,
   label: PropTypes.string,
   description: PropTypes.string,
   onChange: PropTypes.func,