Improve the grader notebooks dropdown

[netoisc]

Update the dropdown style to use a fixed length, to show scroll with long list

[codecov]

Codecov Report

Merging #358 into main will decrease coverage by 0.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #358      +/-   ##
==========================================
- Coverage   79.60%   79.16%   -0.45%     
==========================================
  Files          21       21              
  Lines        1368     1382      +14     
==========================================
+ Hits         1089     1094       +5     
- Misses        279      288       +9     

Impacted Files	Coverage Δ
src/illumidesk/apis/nbgrader_service.py	71.42% <0.00%> (-5.50%)	⬇️
src/illumidesk/grades/senders.py	72.61% <0.00%> (-1.01%)	⬇️
src/illumidesk/lti13/handlers.py	57.14% <0.00%> (ø)
src/illumidesk/spawners/hooks.py	67.74% <0.00%> (ø)
src/illumidesk/apis/jupyterhub_api.py	60.18% <0.00%> (ø)
src/illumidesk/authenticators/constants.py	100.00% <0.00%> (ø)
src/illumidesk/authenticators/validator.py	85.36% <0.00%> (ø)
src/illumidesk/apis/setup_course_service.py	89.28% <0.00%> (ø)
src/illumidesk/authenticators/authenticator.py	92.46% <0.00%> (ø)
src/illumidesk/setup_course/course.py	81.81% <0.00%> (+0.15%)	⬆️

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[jgwerner]

LGTM