individual gradebook: require LMGB flag for outcomes

fixes CNVS-13113

test plan:
  * enable Individual Gradebook View
  * enable Learning Mastery Gradebook
  * open SRGB within a course
  * verify that you have an Outcomes tab presented
  * disable Learning Mastery Gradebook
  * open your browser's traffic log
  * refresh SRGB
  * verify that the tabs are hidden
  * verify that no outcome-related API requests are made

Change-Id: Id432dec32c5f508ed10c6ae1beab84bd4da61588
Reviewed-on: https://gerrit.instructure.com/35163
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>