fix: hidden remove button

[KristinAoki]

Description

This PR fixes the visibility of the "Remove" button for grade segments. Previously, the button would appear on hover above the segment. But the styling of the button blocked it from being visible. Now, the button appears on hover under the grade range. This change impacts Course Authors.

Before

After

Testing instructions

1. Open a course
2. Navigate to the grading page
3. Add a new grade
4. Hover over new grade
5. Confirm that "Remove" is visible
6. Click "Remove"
7. Confirm that the segment is removed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.25%. Comparing base (13b2ed5) to head (445c998).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1600   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files        1100     1100           
  Lines       21794    21794           
  Branches     4702     4701    -1     
=======================================
  Hits        20323    20323           
  Misses       1399     1399           
  Partials       72       72           

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[saadyousafarbi]

PR looks good! Thanks for the fix!