learning mastery student view: outcome hierarchy

fixes CNVS-13307

test plan:
  * enable learning mastery student view
  * open a student's grades page
  * open the learning mastery tab
  * verify that outcome groups are presented in a hierarchy
  * verify that very deep outcome trees are compressed to paths

Change-Id: I1f74dca2876b39b01a6ec70935f60d5e233fe02b
Reviewed-on: https://gerrit.instructure.com/35481
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>