be liberal in checked account namespace

fixes CNVS-17259

some older attachments will have the "accounts_123" style namespace
(local account id), but most newer ones will have the
"accounts_4560000000000123" style namespace (global account id). match
against either when counting attachments for account statistics

test-plan:
 - create an account with a course in it (account statistics don't
   calculate for accounts with zero courses)

 - upload two files in that course

 - in the console, change one attachment's namespace to have the
   account's local ID instead of global ID:
     attachment = Attachment.where(
       namespace: "account_#{account.global_id}").
       first
     attachment.namespace = "account_#{account.local_id}"
     attachment.save!

 - in the console, trigger a run of the statistics aggregator:
     Reporting::CountsReport.process

 - view the account statistics for the account at
   /accounts/:id/statistics; should include both files in the statistics
   (e.g. have 2 files reported, and file size equal to the sum of the
   files sizes)

Change-Id: Ifec4ca47faab89b47244b22f26929cb59e69df92
Reviewed-on: https://gerrit.instructure.com/45636
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>