Expose that sponsor cost is a VAT field in backend interface

While normally sponsors are invoived without VAT, there are cases where
VAT is added and in those cases we want the same VAT preview there as we
have for registration types etc.

The code for dealing with that preview was clearly already designed to
handle this, but it was never enabled for levelcost.

postgresqleu/confsponsor/backendforms.py

@@ -308,6 +308,7 @@ class BackendSponsorshipLevelForm(BackendForm):
     linked_objects = OrderedDict({
         'benefit': BackendSponsorshipLevelBenefitManager(),
     })
+    vat_fields = {'levelcost': 'sponsor'}
     allow_copy_previous = True
     auto_cascade_delete_to = ['sponsorshiplevel_paymentmethods', 'sponsorshipbenefit']
     exclude_date_validators = ['paymentdueby', ]

postgresqleu/confsponsor/migrations/0001_initial.py

@@ -93,7 +93,7 @@ class Migration(migrations.Migration):
                 ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
                 ('levelname', models.CharField(max_length=100, verbose_name='Level name')),
                 ('urlname', models.CharField(max_length=100, validators=[postgresqleu.util.validators.validate_lowercase, postgresqleu.util.validators.validate_urlname], verbose_name='URL name')),
-                ('levelcost', models.IntegerField(verbose_name="Cost")),
+                ('levelcost', models.IntegerField(verbose_name="Cost", help_text='Cost excluding VAT.')),
                 ('available', models.BooleanField(default=True, verbose_name='Available for signup')),
                 ('instantbuy', models.BooleanField(default=False, verbose_name="Instant buy available")),
                 ('canbuyvoucher', models.BooleanField(default=True, verbose_name="Can buy vouchers")),

postgresqleu/confsponsor/models.py

@@ -42,7 +42,7 @@ class SponsorshipLevel(models.Model):
     urlname = models.CharField(max_length=100, null=False, blank=False,
                                validators=[validate_lowercase, validate_urlname],
                                verbose_name="URL name")
-    levelcost = models.IntegerField(null=False, blank=False, verbose_name="Cost")
+    levelcost = models.IntegerField(null=False, blank=False, verbose_name="Cost", help_text="Cost excluding VAT.")
     available = models.BooleanField(null=False, blank=False, default=True, verbose_name="Available for signup")
     public = models.BooleanField(null=False, blank=False, default=True, verbose_name="Publicly visible",
                                  help_text="If unchecked the sponsorship level will be treated as internal, for example for testing")