activity: Use select_related when querying UserActivity records

On my dev machine this cuts /activity load time with lots of users by more than
2/3.  I expect the gains will be even greater in production due to the greater
relative cost of database queries.

(imported from commit 0391cb29f66b618b4b99902d9fb9ab0a6cff0cb3)

zephyr/views.py

@@ -1213,7 +1213,7 @@ class ActivityTable(object):
                 self.has_pointer = True
             for record in UserActivity.objects.filter(
                     query=url,
-                    client__name=client_name):
+                    client__name=client_name).select_related():
                 row = self.rows.setdefault(record.user_profile.user.email, {})
                 row['realm'] = record.user_profile.realm.domain
                 row['full_name'] = record.user_profile.full_name