use a join for course counts on sub accounts

refs CNVS-20225

test plan
 - on sub accounts page the count should be accurate

Change-Id: I6302450ad5ec8c57fbc816ced4d11247abbe4447
Reviewed-on: https://gerrit.instructure.com/53515
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

Author: roor0

app/models/account.rb

@@ -1150,7 +1150,7 @@ def consume_account(account)
   end
 
   def course_count
-    self.child_courses.not_deleted.count('DISTINCT course_id')
+    self.courses.active.count
   end
 
   def sub_account_count