Move file backend to its own service (#9435)

* Move file backend to its own service

* Moving utils/inbucket to mailservice package

app/admin.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/services/mailservice"
 	"github.com/mattermost/mattermost-server/utils"
 )
 
@@ -242,7 +243,7 @@ func (a *App) TestEmail(userId string, cfg *model.Config) *model.AppError {
 	} else {
 		T := utils.GetUserTranslations(user.Locale)
 		license := a.License()
-		if err := utils.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), cfg, license != nil && *license.Features.Compliance); err != nil {
+		if err := mailservice.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), cfg, license != nil && *license.Features.Compliance); err != nil {
 			return model.NewAppError("testEmail", "app.admin.test_email.failure", map[string]interface{}{"Error": err.Error()}, "", http.StatusInternalServerError)
 		}
 	}