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[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/openedx/frontend-app-gradebook/pull/352?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | Coverage Δ | | |---|---|---| | [src/data/thunkActions/roles.js](https://app.codecov.io/gh/openedx/frontend-app-gradebook/pull/352?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx#diff-c3JjL2RhdGEvdGh1bmtBY3Rpb25zL3JvbGVzLmpz) | `100.00% <100.00%> (ø)` | |

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