Django 2.* Model for Global application Settings

admin.py

class SettingsAdmin(admin.ModelAdmin):
    def has_add_permission(self, request):
        # if there's already an entry, do not allow adding
        count = Settings.objects.all().count()
        if count == 0:
            return True
        return False

admin.site.register(Settings, SettingsAdmin)

models.py

class Settings(models.Model):
    site_email = models.EmailField(max_length=255, verbose_name='Основной Email')
    site_phone = models.CharField(max_length=255, verbose_name='Основной телефон')
    site_title = models.CharField(max_length=255, verbose_name='Title сайта', help_text='Текст в тегe <title>')
    site_meta_description = models.CharField(max_length=150, verbose_name='Meta-Description сайта', help_text='max 150 символов')
    site_discount = models.PositiveIntegerField(verbose_name='Размер скидки (в %)', help_text='от 0 до 100', validators=[MinValueValidator(0), MaxValueValidator(100)])

    def __str__(self):
        return 'Настройки сайта'

    class Meta:
        verbose_name = 'Настройки сайта'
        verbose_name_plural = 'Настройки сайта'