Enable freeform-select with fetched column values for filter values (#1697)

* Enable freeform-select with fetched column values for filter values
 - db migration to add filter_select_enabled
 - add freeform-multi option for Selectfield
 - modify formatFilter() function on query to accomodate filter-select

* Fix js tests

* Fix codeclimate issue

* Changes based on comments

* Add test for filter endpoint

* Extract out renderFilterFormField function from render

* Fix landscape issues

superset/assets/spec/javascripts/explorev2/components/Filter_spec.js

@@ -8,7 +8,9 @@ import { shallow } from 'enzyme';
 import Filter from '../../../../javascripts/explorev2/components/Filter';
 
 const defaultProps = {
-  actions: {},
+  actions: {
+    fetchFilterValues: () => ({}),
+  },
   filterColumnOpts: ['country_name'],
   filter: {
     id: 1,