feat: adjusted grade field validation

[Inferato]

TL;DR - added simple validation for the "Adjusted grade" field in the "Edit Grades" popup:

• enter only a numeric value
• setting the minimum value
• checking the correctness of the entered value to possibleGrade

Related: master

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• I've done a visual code review
• I've tested the new functionality

FYI: @openedx/content-aurora

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[e0d]

@Inferato looks like there are some test failures, can you have a look.

[mphilbrick211]

@Inferato looks like there are some test failures, can you have a look.

Hi @Inferato! Just flagging this for you.

[mphilbrick211]

Hi @Inferato! Just following up on the failing check.

[dyudyunov]

@mphilbrick211 @e0d This PR should be closed in favor of #381 where I updated the test snapshot.

[mphilbrick211]

Closing per comment above.

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