[clarity/consistency] rename /explorev2/ -> /explore/ (#2802)

* rename /explorev2/ -> /explore/

* add redirect for existing explorev2 urls

* fix long line

* remove extra line

* fix missed ref in spec

superset/assets/javascripts/explore/components/ControlRow.jsx

@@ -0,0 +1,24 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+
+const NUM_COLUMNS = 12;
+
+const propTypes = {
+  controls: PropTypes.arrayOf(PropTypes.object).isRequired,
+};
+
+function ControlSetRow(props) {
+  const colSize = NUM_COLUMNS / props.controls.length;
+  return (
+    <div className="row space-1">
+      {props.controls.map((control, i) => (
+        <div className={`col-lg-${colSize} col-xs-12`} key={i} >
+          {control}
+        </div>
+      ))}
+    </div>
+  );
+}
+
+ControlSetRow.propTypes = propTypes;
+export default ControlSetRow;