Fix #473 -- Internal name for categories and products (#900)

* Fix #473 -- Internal name for categories and products

* fix pdf renderer

src/pretix/base/migrations/0090_auto_20180509_0917.py

@@ -0,0 +1,29 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-05-09 09:17
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0089_auto_20180315_1322'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='item',
+            name='internal_name',
+            field=models.CharField(blank=True,
+                                   help_text='If you set this, this will be used instead of the public name in the '
+                                             'backend.',
+                                   max_length=255, null=True, verbose_name='Internal name'),
+        ),
+        migrations.AddField(
+            model_name='itemcategory',
+            name='internal_name',
+            field=models.CharField(blank=True,
+                                   help_text='If you set this, this will be used instead of the public name in the backend.',
+                                   max_length=255, null=True, verbose_name='Internal name'),
+        ),
+    ]