feat: added ORA grade assigned notification

[eemaanamir]

Ticket: INF-1538

Implemented notification type ora_grade_assigned sent to the student immediately after a final grade has been assigned to their Open Response Assessment.

Backend PR

[codecov]

Codecov Report

Attention: Patch coverage is 98.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.12%. Comparing base (8dbf14c) to head (0d11481).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
openassessment/workflow/models.py	80.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2232      +/-   ##
==========================================
+ Coverage   95.10%   95.12%   +0.01%     
==========================================
  Files         195      195              
  Lines       21485    21601     +116     
  Branches     1931     1936       +5     
==========================================
+ Hits        20433    20547     +114     
  Misses        786      786              
- Partials      266      268       +2     

Flag	Coverage Δ
unittests	95.12% <98.33%> (+0.01%)	⬆️

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