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/urls.py

@@ -34,6 +34,7 @@ urlpatterns = [
 
     # ui views
     re_path(r'^login/?$', views.login_page),
+    re_path(r'^about/?$', views.about_page),
 
     path('', views.home),
     re_path(r'^(?P<tab>home|local|federated)/?$', views.home_tab),