Feature: Disable authentication for contributors

[nick-y-ito]

Overview

This PR disables the authentications for contributors to work in their local environment.

Changes

• Disabled the "Sign-In" flow if working in the local environment and the environment variables for OAuth are not defined.
• Disabled the JWT token if the API server that is configured in the .env.local is the mock API server.
• Add postinstall script to automatically copy the .env.example to .env.local if not existing

Review points

• Do they work as expected?
• Is there any security vulnerability?

Screen Captures

Notes

• The login page is still reachable by directly accessing /login even if the login flow is disabled.
• "Sign-in" and "token check on API request" can be enabled/disabled individually.

Assignee Checklist:

• [x] The naming convention of the PR title is correct (See the comment at the top of this template)
• [x] The base branch is correct (See: Types of Branches)
• [x] The branch name follows the Branch Naming Conventions
• [x] The correct assignees and reviewers have been designated for this PR
• [x] The coding style follows the Coding Style Guide
• [x] All the related issues are associated with this PR
• [x] All criteria in the associated issues are met (please tick the checkboxes)
• [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]

@kotaaaa Thank you for your review and approval!

After our conversation, I decided to stick to this code. However, I appreciated learning your perspective. https://github.com/nishiki-tech/nishiki-frontend/pull/290#discussion_r1508050855