Alerting: Add external Alertmanagers (#39183)

* building ui

* saving alertmanager urls

* add actions and api call to get external ams

* add list to add modal

* add validation and edit/delete

* work on merging results

* merging results

* get color for status heart

* adding tests

* tests added

* rename

* add pollin and status

* fix list sync

* fix polling

* add info icon with actual tooltip

* fix test

* Accessibility things

* fix strict error

* delete public/dist files

* Add API tests for invalid URL

* start redo admin test

* Fix for empty configuration and test

* remove admin test

* text updates after review

* suppress appevent error

* fix tests

* update description

Co-authored-by: gotjosh <josue@grafana.com>

* fix text plus go lint

* updates after pr review

* Adding docs

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* prettier

* updates after docs feedback

Co-authored-by: gotjosh <josue.abreu@gmail.com>
Co-authored-by: gotjosh <josue@grafana.com>

pkg/services/ngalert/api/api_admin.go

@@ -70,6 +70,12 @@ func (srv AdminSrv) RoutePostNGalertConfig(c *models.ReqContext, body apimodels.
 		OrgID:         c.OrgId,
 	}
 
+	if err := cfg.Validate(); err != nil {
+		msg := "failed to validate admin configuration"
+		srv.log.Error(msg, "err", err)
+		return ErrResp(http.StatusBadRequest, err, msg)
+	}
+
 	cmd := store.UpdateAdminConfigurationCmd{AdminConfiguration: cfg}
 	if err := srv.store.UpdateAdminConfiguration(cmd); err != nil {
 		msg := "failed to save the admin configuration to the database"