DB: Identifiers of courses are scoped to semester

Closes #34.

db.ddl

@@ -27,11 +27,12 @@ CREATE TABLE owl_semesters (
 CREATE TABLE owl_courses (
 	cid		serial		PRIMARY KEY,
 	semid		int		NOT NULL REFERENCES owl_semesters(semid) ON DELETE CASCADE,
-	ident		varchar(255)	UNIQUE NOT NULL,
+	ident		varchar(255)	NOT NULL,
 	name		varchar(255)	NOT NULL,
 	enroll_token	varchar(255)	UNIQUE NOT NULL,
 	student_grading	boolean		NOT NULL DEFAULT FALSE,
-	anon_grading	boolean		NOT NULL DEFAULT FALSE
+	anon_grading	boolean		NOT NULL DEFAULT FALSE,
+	UNIQUE(semid, cid)
 );
 
 CREATE TABLE owl_enroll (