Uses fixed string as site name

The domain is *better* but it was causing the makemigrations to trigger
whenever the domain name in .env changes, which is a real hassle with
a free version of ngrok

bookwyrm/migrations/0051_auto_20201005_2142.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-10-05 21:42
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0050_auto_20201002_2156'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='sitesettings',
+            name='name',
+            field=models.CharField(default='BookWyrm', max_length=100),
+        ),
+    ]