import points_possible from CC 1.3 assignments

test plan:
 - take the flat manifest attached to CNVS-13219 and find the <gradable>
   tag
 - add a points_possible attribute to it, for example
   <gradable points_possible="101">
 - import the manifest as a common cartridge
 - the imported assignment should have the specified points possible

refs CNVS-13503
fixes CNVS-13576

Change-Id: Ifc546c47cada7fcf7ec817395ceb525028406bf1
Reviewed-on: https://gerrit.instructure.com/36269
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>