Email: Use MJML email templates (#57751)

Co-authored-by: Santiago <santiagohernandez.1997@gmail.com>

pkg/services/ngalert/notifier/channels/email_test.go

@@ -189,9 +189,13 @@ func TestEmailNotifierIntegration(t *testing.T) {
 			messageTmpl: "",
 			expSubject:  "[FIRING:2]  ",
 			expSnippets: []string{
-				"Firing: 2 alerts",
-				"<li>alertname: FiringOne</li><li>severity: warning</li>",
-				"<li>alertname: FiringTwo</li><li>severity: critical</li>",
+				"2 firing instances",
+				"<strong>severity</strong>",
+				"warning\n",
+				"critical\n",
+				"<strong>alertname</strong>",
+				"FiringTwo\n",
+				"FiringOne\n",
 				"<a href=\"http://fix.me\"",
 				"<a href=\"http://localhost/base/d/abc",
 				"<a href=\"http://localhost/base/d/abc?viewPanel=5",

pkg/services/notifications/notifications.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/Masterminds/sprig/v3"
 	"github.com/grafana/grafana/pkg/bus"
 	"github.com/grafana/grafana/pkg/events"
 	"github.com/grafana/grafana/pkg/infra/log"
@@ -53,6 +54,7 @@ func ProvideService(bus bus.Bus, cfg *setting.Cfg, mailer Mailer, store TempUser
 	mailTemplates.Funcs(template.FuncMap{
 		"Subject": subjectTemplateFunc,
 	})
+	mailTemplates.Funcs(sprig.FuncMap())
 
 	for _, pattern := range ns.Cfg.Smtp.TemplatesPatterns {
 		templatePattern := filepath.Join(ns.Cfg.StaticRootPath, pattern)