updated learning outcome to retain eval method and info for weighted methods

fixes CNVS-16915

(all mentions of 'gradebook' refer to learning mastery gradebook)
test plan:
- go to gradebook
- all existing mastery scores should still default to highest score to retain
  accuracy of existing calculations
- in rails console, set 'calculation_method' to 'latest' for an outcome
- all results for that outcome should now show student's latest score on gradebook
- in rails console, set method to 'highest'
- all results for that outcome should now show student's highest score on gradebook
- in rails console, set method to 'n_mastery' and set 'calculation_int' to a number
  between two and 5
- all results for that outcome should now show avg of student's higest n# of scores
  (#n being the number set for the outcome's 'calculation_int')
- in rails console, set method to 'decaying_average' and set 'calculation_int' to 75
- scores should now appear as they did first time gradebook was viewed on step #2
- in rails console, set 'calculation_int' to 65
- scores for that outcome should now reflect decaying average with 65% weight given to
  most recent score, and 35% weight for avg of all prior scores
- have a student make a new submission to an old assignment and give it a score
- the re-submission should now be most recent score for 'latest' and
  'decaying_average' methods
- amend a score for an old submission without a new submission from the student
- the amended score should not become the most recent, but still be accurately
  reflected in the 25%/35% part of the 'decaying_average' method

Change-Id: I330c47cb2288d99e589deca00ddb5a8ee8103a91
Reviewed-on: https://gerrit.instructure.com/44935
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Benjamin Porter <bporter@instructure.com>