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/__init__.py

@@ -6,3 +6,4 @@ from .status import Favorite, Boost, Tag, Notification, ReadThrough
 from .user import User, UserFollows, UserFollowRequest, UserBlocks
 from .user import FederatedServer
 from .import_job import ImportJob, ImportItem
+from .site import SiteSettings