fix rounding to whole number
fix rounding to whole number

In Gradebook, the Assignment Details modal should show a decimal
average computed for the assignment. Previously, it rounded to a
whole number. Now it rounds to 2 decimal places.

fixes CNVS-15945

test plan:
1. Create an assignment (publish it)
2. Go to the gradebook and enter grades with one and
   two decimal places (e.g 5.5, 5.35)
3. hover over the assignment title
4. click on dropdown and click on assignment details
5. notice that the average score is rounded up/down
6. it should now display proper decimal rounding

Change-Id: I56d8600aab3b5184c494b1f32889cd4d231d0148
Reviewed-on: https://gerrit.instructure.com/78252
Reviewed-by: Shahbaz Javeed <6692d7be3fa215e5483a121d6fb314def6049877@instructure.com>
Reviewed-by: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
Tested-by: Jenkins
QA-Review: KC Naegle <9fbecd016338f9910821a8a5d5d0ae4fc9abfaa5@instructure.com>
Product-Review: Christi Wruck

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