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/models/site.py

@@ -11,7 +11,7 @@ from .user import User
 
 class SiteSettings(models.Model):
     ''' customized settings for this instance '''
-    name = models.CharField(default=DOMAIN, max_length=100)
+    name = models.CharField(default='BookWyrm', max_length=100)
     instance_description = models.TextField(
         default="This instance has no description.")
     code_of_conduct = models.TextField(