Explore: time selector

* time selector for explore section
* mostly ported the angular time selector, but left out the timepicker
 (3rd-party angular component)
* can be initialised via url parameters (jump from panels to explore)
* refreshing not implemented for now
* moved the forward/backward nav buttons around the time selector

public/app/features/panel/metrics_panel_ctrl.ts

@@ -324,7 +324,12 @@ class MetricsPanelCtrl extends PanelCtrl {
   }
 
   explore() {
-    const exploreState = encodePathComponent(JSON.stringify(this.datasource.getExploreState(this.panel)));
+    const range = this.timeSrv.timeRangeForUrl();
+    const state = {
+      ...this.datasource.getExploreState(this.panel),
+      range,
+    };
+    const exploreState = encodePathComponent(JSON.stringify(state));
     this.$location.url(`/explore/${exploreState}`);
   }