Original price for variations (#1258)

* Original price for variations

* Documentation

* API-GET

* Fix existing tests to accomodate new attribute

* Test for variation's original_price on API

src/pretix/base/migrations/0117_auto_20190418_1149.py

@@ -0,0 +1,23 @@
+# Generated by Django 2.2 on 2019-04-18 11:49
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0116_auto_20190402_0722'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='itemvariation',
+            name='original_price',
+            field=models.DecimalField(blank=True, decimal_places=2, help_text='If set, this will be displayed next to '
+                                                                              'the current price to show that the '
+                                                                              'current price is a discounted one. '
+                                                                              'This is just a cosmetic setting and '
+                                                                              'will not actually impact pricing.',
+                                      max_digits=7, null=True, verbose_name='Original price'),
+        ),
+    ]