Add instance settings.

Addresses: #58

Currently implemented:

* Instance name
* Instance description
* Code of conduct
* Allow registration

I decided to store this in the database so that settings can be easily
changed at runtime through the web interface. (This web interface does
not exist.)

fedireads/models/site.py

@@ -0,0 +1,19 @@
+from django.db import models
+from fedireads.settings import DOMAIN
+
+class SiteSettings(models.Model):
+    name = models.CharField(default=DOMAIN, max_length=100)
+    instance_description = models.TextField(
+        default="This instance has no description.")
+    code_of_conduct = models.TextField(
+        default="Add a code of conduct here.")
+    allow_registration = models.BooleanField(default=True)
+
+    @classmethod
+    def get(cls):
+        try:
+            return cls.objects.get(id=1)
+        except cls.DoesNotExist:
+            default_settings = SiteSettings(id=1)
+            default_settings.save()
+            return default_settings