Join: If already joined, redirect to course

Closes #106.

owl/course.py

@@ -179,6 +179,9 @@ def enroll_token(token: str) -> str:
         if form.validate_on_submit():
             enroll_commit(course)
             return redirect(url_for('course_index', sident=course.semester.ident, cident=course.ident))
+    elif course is not None:
+        # Already joined
+        return redirect(url_for('course_index', sident=course.semester.ident, cident=course.ident))
 
     return render_template('join-token.html', form=form, error=err, course=course)