[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/controls/HiddenControl.jsx

@@ -0,0 +1,23 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { FormControl } from 'react-bootstrap';
+
+const propTypes = {
+  onChange: PropTypes.func,
+  value: PropTypes.oneOfType([
+    PropTypes.string,
+    PropTypes.number,
+  ]),
+};
+
+const defaultProps = {
+  onChange: () => {},
+};
+
+export default function HiddenControl(props) {
+  // This wouldn't be necessary but might as well
+  return <FormControl type="hidden" value={props.value} />;
+}
+
+HiddenControl.propTypes = propTypes;
+HiddenControl.defaultProps = defaultProps;