Explore: Add entry to panel menu to jump to Explore

* panel container menu gets new Explore entry (between Edit and Share)
* entry only shows if datasource has `supportsExplore` set to true (set
 for Prometheus only for now)
* click on Explore entry changes url to `/explore/state` via location provider
* `state` is a JSON representation of the panel queries
* datasources implement `getExploreState()` how to turn a panel config into explore initial
 state
* Explore can parse the state and initialize its query expressions
* ReactContainer now forwards route parameters as props to component
* `pluginlist` and `singlestat` panel subclasses needed to be adapted because
 `panel_ctrl` now has the location provider as a property already

public/app/routes/ReactContainer.tsx

@@ -29,6 +29,7 @@ export function reactContainer($route, $location, backendSrv: BackendSrv, dataso
       const props = {
         backendSrv: backendSrv,
         datasourceSrv: datasourceSrv,
+        routeParams: $route.current.params,
       };
 
       ReactDOM.render(WrapInProvider(store, component, props), elem[0]);