dashboards: add support for retrieving a dashboard by uid

Introduces new url in api /dashboards/<uid> for fetching dashboard by unique id
Leave the old dashboard by slug url /dashboards/db/<slug> for backward
compatibility and for supporting fallback
WIP for #7883

pkg/api/api.go

@@ -242,6 +242,8 @@ func (hs *HttpServer) registerRoutes() {
 
 		// Dashboard
 		apiRoute.Group("/dashboards", func(dashboardRoute RouteRegister) {
+			dashboardRoute.Get("/uid/:uid", wrap(GetDashboard))
+
 			dashboardRoute.Get("/db/:slug", wrap(GetDashboard))
 			dashboardRoute.Delete("/db/:slug", reqEditorRole, wrap(DeleteDashboard))