fix: add limited_staff to allowedRoles [BB-7834]

[0x29a]

Description

Users with the Course Limited Staff role, that has been added here, don't have access to the gradebook. In this PR we add this role's slug to allowedRoles.

Testing instructions

The fix is pretty trivial, so just make sure that the slug in allowedRoles is equal to CourseLimitedStaffRole.ROLE value.

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (5e96dbf) 94.93% compared to head (1514774) 94.93%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #352   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         139      139           
  Lines        1343     1343           
  Branches      264      264           
=======================================
  Hits         1275     1275           
  Misses         60       60           
  Partials        8        8           

Files Changed	Coverage Δ
src/data/thunkActions/roles.js	100.00% <100.00%> (ø)

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