Explore: jump to explore from panels with mixed datasources

- extends handlers for panel menu and keypress 'x'
- in a mixed-datasource panel finds first datasource that supports
  explore and collects its targets
- passes those targets to the found datasource to be serialized for
  explore state
- removed `supportMetrics` and `supportsExplore`
- use datasource metadata instead (set in plugin.json)
- Use angular timeout to wrap url change for explore jump
- Extract getExploreUrl into core/utils/explore

public/app/plugins/datasource/prometheus/datasource.ts

@@ -149,8 +149,6 @@ export class PrometheusDatasource {
   editorSrc: string;
   name: string;
   ruleMappings: { [index: string]: string };
-  supportsExplore: boolean;
-  supportMetrics: boolean;
   url: string;
   directUrl: string;
   basicAuth: any;
@@ -166,8 +164,6 @@ export class PrometheusDatasource {
     this.type = 'prometheus';
     this.editorSrc = 'app/features/prometheus/partials/query.editor.html';
     this.name = instanceSettings.name;
-    this.supportsExplore = true;
-    this.supportMetrics = true;
     this.url = instanceSettings.url;
     this.directUrl = instanceSettings.directUrl;
     this.basicAuth = instanceSettings.basicAuth;
@@ -522,10 +518,10 @@ export class PrometheusDatasource {
     });
   }
 
-  getExploreState(panel) {
+  getExploreState(targets: any[]) {
     let state = {};
-    if (panel.targets) {
-      const queries = panel.targets.map(t => ({
+    if (targets && targets.length > 0) {
+      const queries = targets.map(t => ({
         query: this.templateSrv.replace(t.expr, {}, this.interpolateQueryExpr),
         format: t.format,
       }));