feat: update privacy policy URL and add validation for privacy policy link

[realethanhsu]

… policy link

Summary

1. Fix Dify's privacy policy URL.

2. Add a new feature to check whether a privacy policy URL is valid.

Tip

Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

Screenshots

Before

After set privacy policy as test privacy policy, and open it in webapp.

After

When we submit the information of app site, it will check whether a privacy policy URL is valid.

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods