Extracting mail service into shared libs (#17030)

* Extracting mail service into shared libs

* Fixing golangci-lint

app/admin.go

@@ -14,8 +14,8 @@ import (
 	"time"
 
 	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/services/mailservice"
 	"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/utils"
 )
@@ -228,7 +228,7 @@ func (a *App) TestEmail(userID string, cfg *model.Config) *model.AppError {
 	T := i18n.GetUserTranslations(user.Locale)
 	license := a.Srv().License()
 	mailConfig := a.Srv().MailServiceConfig()
-	if err := mailservice.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, ""); err != nil {
+	if err := mail.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, ""); err != nil {
 		return model.NewAppError("testEmail", "app.admin.test_email.failure", map[string]interface{}{"Error": err.Error()}, "", http.StatusInternalServerError)
 	}