Update URLs to demo-data repo to use GatherPress namespace

[carstingaxion]

Description of the Change

Closes #649, Closes #663

How to test the Change

Changelog Entry

Changed - Update URLs to demo-data repo to use GatherPress namespace

Credits

Props @carstingaxion

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[what-the-diff]

PR Summary

• Updated Demo Data Import URL in create-preview-links.js
  The location from where the application fetches demo data (used for testing and presentation purposes) has been updated in the file create-preview-links.js. The new location ensures data integrity as it is now fetched from the official Gatherpress repository instead of an individual user's repository.

• Updated Demo Data Import URL in blueprint.json
  Similar to the earlier point, the data import URL for demo data in the file blueprint.json has been updated as well. This change too improves data security and reduces dependency on a singular user's repository.