Cannot edit space templates if they have changes in the global template version

Issue Summary

When a global template has been changed, we are not able to edit the template at the space-level

Steps to Reproduce

1. Navigate to Settings > Global Templates and Blueprints;
2. Scroll down to the Meeting Notes template and edit it (not the index one);
3. Add any changes to it and save it;
4. Navigate to any space you are a space admin and access Space Settings > Content Tools > Templates;
5. Scroll down to the Meeting Notes template and edit it (not the index one);
6. Add any changes to it and hit the Save button.

Expected Results

The changes are saved as usual.

Actual Results

An error is thrown in the screen saying "Unable to communicate with server. Saving is not possible at the moment."

Notes

• After hitting the save button, a 500 error occurs in the saving endpoint https://dorime.atlassian.net/wiki/rest/api/template with the below response:

  {
      "statusCode": 500,
      "message": "javax.persistence.PersistenceException: org.hibernate.HibernateException: Found shared references to a collection: com.atlassian.confluence.pages.templates.PageTemplate.templateAttachments"
  }
  

• This is also reproducible with some other templates;
• If you have edited your space template prior to the global template changes, you will be able to keep editing it, as long as you do not reset it to default.

Workaround

For now, you can:

• Edit the space template, copy its content, and create a new custom template to add your changes there adn use it while we work to fix this issue;
• Reset the global template.