Refactor grade table submission via xlsx

What does this MR do?

This MR refactors the teacher's exam grade table submission.

Basic UI:

Initial submission (signing process):

Remote signature (cloud) signing:

Validation results with invalid grades (which always come first at the list so they can be easily spotted):

Validation results with only valid grades (notice that the final submission button is now enabled):

This MR also closes #313 (closed) . See universis!871 (merged) for more info.

Related issues and links

Fix #303 (closed)

Other relevant links

Developer Checklist

• I have successfully run the code of this merge request locally

• I have verified locally that my changes work for all necessary screen sizes

• I have tried out the changes of this MR with different users to identify bugs

• Coding is in progress, and I have marked the MR as WIP

• Coding is completed and the MR is ready for review

• My branch is up-to-date with the upstream master branch

• My MR follows the contribution guidelines

• I have added a comment with screenshots of the code running locally

Tech review Checklist

Have you verified that what is supposed to happen, actually does, and what is not supposed to happen, indeed does not?

• The MR accurately describes the changes and has a relevant title/description
• The MR does what it is supposed to according to its title, description and related issues/links
• I have successfully run the changes locally, and tried the new code

Overall review Checklist

• The MR references related issues/MRs
• The MR provides links to screens and screenshots
• The commits of the MR describe the changes, have proper wording, and follow the guidelines
• I have successfully run the changes locally, and tried the new code
• The MR is ready for merge (rebased, commit squashed if needed, etc)