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/javascripts/explorev2/components/ControlPanelsContainer.jsx

@@ -102,6 +102,7 @@ class ControlPanelsContainer extends React.Component {
                 filters={this.props.form_data.filters}
                 actions={this.props.actions}
                 prefix={section.prefix}
+                datasource_id={this.props.form_data.datasource}
               />
             </ControlPanelSection>
           ))}