Duplicated confirm button at top roster upload confirmation screen

[michellexliu]

Description

Added a second "Confirm" button at the top of the student table on the roster upload confirmation screen.

Motivation and Context

Suggested by Iliano so that professors don't need to scroll all the way to the bottom of the screen for large classes.

How Has This Been Tested?

Clicked the new button to check that

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting
• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

[victorhuangwq]

Confirm button appears at the top. LGTM