Fix: Add loading state to each submit button

[nick-y-ito]

Overview

This PR is a takeover of https://github.com/nishiki-tech/nishiki-frontend/pull/368. Please take a look at the original PR for details.

Review points

• I would like you to double-check if each form only allows one submission. Multiple submissions used to be possible by either mashing the submit button of the UI or the [ enter ] key.

Notes

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Assignee Checklist:

• [x] The naming convention of the PR title is correct (See the comment at the top of this template)
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• [x] The coding style follows the Coding Style Guide
• [x] All the related issues are associated with this PR
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• [x] My changes do not generate new warnings or errors (especially in the console)

Reviewer Checklist:

• [ ] Double-check the "Assignee Checklist"
• [ ] The code follows the generic best practices and our Coding Style Guide
• [ ] The code is well-commented and easy to understand
• [ ] The UI changes accurately reflect the provided design

[nick-y-ito]

@ShoeheyOt @kanta1207 Could you review this PR when you have time, please?

[nick-y-ito]

@ShoeheyOt Thank you for your review :) I will merge this PR.