Explore Datasource selector

Adds a datasource selector to the Explore UI. Only datasource plugins
that have `explore: true` in their `plugin.json` can be selected.

- adds datasource selector (based on react-select) to explore UI
- adds getExploreSources to datasource service
- new `explore` flag in datasource plugins model
- Prometheus plugin enabled explore

public/app/plugins/datasource/prometheus/plugin.json

@@ -2,21 +2,30 @@
   "type": "datasource",
   "name": "Prometheus",
   "id": "prometheus",
-
   "includes": [
-    {"type": "dashboard", "name": "Prometheus Stats", "path": "dashboards/prometheus_stats.json"},
-    {"type": "dashboard", "name": "Prometheus 2.0 Stats", "path": "dashboards/prometheus_2_stats.json"},
-    {"type": "dashboard", "name": "Grafana Stats", "path": "dashboards/grafana_stats.json"}
+    {
+      "type": "dashboard",
+      "name": "Prometheus Stats",
+      "path": "dashboards/prometheus_stats.json"
+    },
+    {
+      "type": "dashboard",
+      "name": "Prometheus 2.0 Stats",
+      "path": "dashboards/prometheus_2_stats.json"
+    },
+    {
+      "type": "dashboard",
+      "name": "Grafana Stats",
+      "path": "dashboards/grafana_stats.json"
+    }
   ],
-
   "metrics": true,
   "alerting": true,
   "annotations": true,
-
+  "explore": true,
   "queryOptions": {
     "minInterval": true
   },
-
   "info": {
     "description": "Prometheus Data Source for Grafana",
     "author": {
@@ -28,8 +37,11 @@
       "large": "img/prometheus_logo.svg"
     },
     "links": [
-      {"name": "Prometheus", "url": "https://prometheus.io/"}
+      {
+        "name": "Prometheus",
+        "url": "https://prometheus.io/"
+      }
     ],
     "version": "5.0.0"
   }
-}
+}