[explore-v2] control panel fixes (#1529)

* make fieldset conditions more clear

* make label required

* use render* pattern

* use slugify util for turning labels into ids

* use field rather than html

* don't need panel-title class here

caravel/assets/javascripts/explorev2/components/ControlLabelWithTooltip.jsx

@@ -3,12 +3,11 @@ import { ControlLabel } from 'react-bootstrap';
 import InfoTooltipWithTrigger from '../../components/InfoTooltipWithTrigger';
 
 const propTypes = {
-  label: PropTypes.string,
+  label: PropTypes.string.isRequired,
   description: PropTypes.string,
 };
 
 const defaultProps = {
-  label: null,
   description: null,
 };