Rearrange and update API throttles

gracedb/api/v1/events/throttles.py → gracedb/api/v1/events/throttling.py

-from ..throttles import PostOrPutUserRateThrottle
+from api.throttling import PostOrPutUserRateThrottle
 
 
 class EventCreationThrottle(PostOrPutUserRateThrottle):

gracedb/api/v2/events/throttles.py

-from ...v1.events.throttles import *

gracedb/api/v2/events/throttling.py

+from ...v1.events.throttling import *

gracedb/api/v2/throttles.py

-from ..v1.throttles import *