Add expiry dates and individual conditions to gift cards (#1656)

* Add expiry dates and individual conditions to gift cards

* Display refund gift cards with more details and prettier interface

* Allow to set gift card expiry and conditions when cancelling event

* Extend gift card search

* Fix #1565 -- Some gift card filters

* Improve list of gift cards

* Allow to edit gift cards

* Note on validity

src/pretix/base/migrations/0151_auto_20200421_0737.py

@@ -0,0 +1,26 @@
+# Generated by Django 3.0.5 on 2020-04-21 07:37
+
+import django_countries.fields
+from django.db import migrations, models
+
+import pretix.helpers.countries
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0150_auto_20200401_1123'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='giftcard',
+            name='conditions',
+            field=models.TextField(null=True),
+        ),
+        migrations.AddField(
+            model_name='giftcard',
+            name='expires',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]