Add blank=True for Post.user_token

This was preventing tests from passing and having a blanked-out user
token, which is OK

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

board/migrations/0005_alter_post_user_token.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.1b1 on 2022-07-24 00:21
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("board", "0004_board_spoilers"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="post",
+            name="user_token",
+            field=models.CharField(blank=True, max_length=30, null=True),
+        ),
+    ]