dashboards: add new default frontend route for loading a dashboard

New default route /d/<uid>/<slug of dashboard title> where dashboard
are loaded by unique identifier.
If old route /dashboard/db/<slug of dashboard tile> are used,
try to lookup dashboard by slug and redirect to new default route.
 #7883

public/app/core/services/backend_srv.ts

@@ -225,6 +225,10 @@ export class BackendSrv {
     return this.get('/api/dashboards/' + type + '/' + slug);
   }
 
+  getDashboardByUid(uid: string) {
+    return this.get(`/api/dashboards/uid/${uid}`);
+  }
+
   saveDashboard(dash, options) {
     options = options || {};