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/index.jsx

@@ -19,6 +19,7 @@ const bootstrappedState = Object.assign(initialState, {
   datasource_type: bootstrapData.datasource_type,
   viz: bootstrapData.viz,
 });
+
 const store = createStore(exploreReducer, bootstrappedState,
   compose(applyMiddleware(thunk))
 );