enforce unique content generator names in database

Set a uniqueness constraint on the content generator name in the
database.

Prior to this change, we were only enforcing this in the hub application
layer. Configure this in postgres for safety.

docs/schema.sql

@@ -257,7 +257,7 @@ INSERT INTO volume (id, name) VALUES (0, 'DEFAULT');
 -- data for content generators
 CREATE TABLE content_generator (
 	id SERIAL PRIMARY KEY,
-	name TEXT
+	name TEXT UNIQUE NOT NULL
 ) WITHOUT OIDS;
 
 -- here we track the built packages