Store organisation type against each service

> So that we can default services to their appropriate text allowance,
> we need to find out what sector they're in. So let's start collecting
> that from teams as they create new services.

In order to work out what a team’s allowance should be, we need to know
what part of government they’re from. We’re going to do this logic in
the admin app and then `POST` the allowance to the API.

So all we need to do with the information that the users give us is
store it against the service, so we have a record. Doesn’t need any
logic doing as a result of it, doesn’t need foreign keying to the
organisations table, etc.

app/models.py

@@ -228,6 +228,10 @@ class Service(db.Model, Versioned):
         nullable=False,
         default=BRANDING_GOVUK
     )
+    organisation_type = db.Column(
+        db.String(255),
+        nullable=True,
+    )
 
     association_proxy('permissions', 'service_permission_types')