Fix calculation of total points in exports and API

Previously, multiple awards in a thread were added instead of the
last one taking precedence.

Also, point awards in global posts are no longer permitted,
so we can simiplify the code.

owl/api.py

@@ -266,19 +266,15 @@ def get_points(course: db.Course, students: List[db.User]) -> Tuple[Dict[int, Di
     ))
 
     points: Dict[int, Dict[str, float]] = {s.uid: {} for s in students}
-    total_points: Dict[int, float] = {s.uid: 0 for s in students}
 
     for a in awards:
         pts = float(a.points)
-        if a.target_uid == -1:
-            for uid in points.keys():
-                points[uid][a.topic.ident] = pts
-                if not a.topic.standalone:
-                    total_points[uid] += pts
-        elif a.target_uid in points:
+        if a.target_uid in points:
             points[a.target_uid][a.topic.ident] = pts
-            if not a.topic.standalone:
-                total_points[a.target_uid] += pts
+
+    total_points: Dict[int, float] = {}
+    for s in students:
+        total_points[s.uid] = sum(points[s.uid].values())
 
     return points, total_points