Extracting mail service into shared libs (#17030)

* Extracting mail service into shared libs

* Fixing golangci-lint

app/app.go

@@ -15,10 +15,10 @@ import (
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/services/httpservice"
 	"github.com/mattermost/mattermost-server/v5/services/imageproxy"
-	"github.com/mattermost/mattermost-server/v5/services/mailservice"
 	"github.com/mattermost/mattermost-server/v5/services/searchengine"
 	"github.com/mattermost/mattermost-server/v5/services/timezones"
 	"github.com/mattermost/mattermost-server/v5/shared/i18n"
+	"github.com/mattermost/mattermost-server/v5/shared/mail"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
 	"github.com/mattermost/mattermost-server/v5/shared/templates"
 	"github.com/mattermost/mattermost-server/v5/utils"
@@ -498,7 +498,7 @@ func (a *App) NotifyAndSetWarnMetricAck(warnMetricId string, sender *model.User,
 				return model.NewAppError("NotifyAndSetWarnMetricAck", "api.email.send_warn_metric_ack.failure.app_error", map[string]interface{}{"Error": err.Error()}, "", http.StatusInternalServerError)
 			}
 
-			if err := mailservice.SendMailUsingConfig(model.MM_SUPPORT_ADVISOR_ADDRESS, subject, body, mailConfig, false, sender.Email); err != nil {
+			if err := mail.SendMailUsingConfig(model.MM_SUPPORT_ADVISOR_ADDRESS, subject, body, mailConfig, false, sender.Email); err != nil {
 				return model.NewAppError("NotifyAndSetWarnMetricAck", "api.email.send_warn_metric_ack.failure.app_error", map[string]interface{}{"Error": err.Error()}, "", http.StatusInternalServerError)
 			}
 		}