Update ansible playbook and constants to remove nbgrader configuration

[jgwerner]

• Remove nbgrader role form ansible playbook
• Add constant for course config
• Update tests

#235

[codecov]

Codecov Report

Merging #236 into main will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #236      +/-   ##
==========================================
+ Coverage   80.75%   80.92%   +0.16%     
==========================================
  Files          19       19              
  Lines        1159     1169      +10     
==========================================
+ Hits          936      946      +10     
  Misses        223      223              

Impacted Files	Coverage Δ
src/illumidesk/setup_course/constants.py	100.00% <100.00%> (ø)
src/illumidesk/setup_course/course.py	82.82% <100.00%> (+1.71%)	⬆️

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[netoisc]

I wonder if this shouldn't have been in another new branch to definitely deprecate all things related to the Firstuse/grader service. But the changes seem all ok