Alerting: Remove url based external alertmanagers config (#57918)

* Remove URL-based alertmanagers from endpoint config

* WIP

* Add migration and alertmanagers from admin_configuration

* Empty comment removed

* set BasicAuth true when user is present in url

* Remove Alertmanagers from GET /admin_config payload

* Remove URL-based alertmanager configuration from UI

* Fix new uid generation in external alertmanagers migration

* Fix tests for URL-based external alertmanagers

* Fix API tests

* Add more tests, move migration code to separate file, and remove possible am duplicate urls

* Fix edge cases in migration

* Fix imports

* Remove useless fields and fix created_at/updated_at retrieval

Co-authored-by: George Robinson <george.robinson@grafana.com>
Co-authored-by: Konrad Lalik <konrad.lalik@grafana.com>

pkg/services/ngalert/models/admin_configuration.go

@@ -1,10 +1,7 @@
 package models
 
 import (
-	"crypto/sha256"
 	"errors"
-	"fmt"
-	"net/url"
 )
 
 type AlertmanagersChoice int
@@ -26,9 +23,6 @@ type AdminConfiguration struct {
 	ID    int64 `xorm:"pk autoincr 'id'"`
 	OrgID int64 `xorm:"org_id"`
 
-	// List of Alertmanager(s) URL to push alerts to.
-	Alertmanagers []string
-
 	// SendAlertsTo indicates which set of alertmanagers will handle the alert.
 	SendAlertsTo AlertmanagersChoice `xorm:"send_alerts_to"`
 
@@ -36,23 +30,6 @@ type AdminConfiguration struct {
 	UpdatedAt int64 `xorm:"updated"`
 }
 
-func (ac *AdminConfiguration) AsSHA256() string {
-	h := sha256.New()
-	_, _ = h.Write([]byte(fmt.Sprintf("%v", ac.Alertmanagers)))
-	return fmt.Sprintf("%x", h.Sum(nil))
-}
-
-func (ac *AdminConfiguration) Validate() error {
-	for _, u := range ac.Alertmanagers {
-		_, err := url.Parse(u)
-		if err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
 // String implements the Stringer interface
 func (amc AlertmanagersChoice) String() string {
 	return alertmanagersChoiceMap[amc]