WIP: Resolve "Project custom file templates"

What does this MR do?

Add custom file templates with the following architecture:

└── templates
    └── personalised_templates
        ├── Documents
        │   ├── blog-post.md
        │   └── release_notes.md
        │   └── some_weird_kind_of_json_document.json
        └── Configuration
            ├── _config.yaml
            └── _items.json

From Gitlab Foss MR: gitlab-foss!31016 (closed)

Still pending issues from previous MR would be:

• the integration of the templates in the WebIDE, as there are no categories for templates selection in the second dropdown of the WebIDE contrary to the normal editor.
• the link to retrieve the templates with the API, to create a nicer link to retrieve the file like is done for issues or MRs (from /api/v4 /projects/19/repository/files/.gitlab/personalised_templates/Documents/blog-post.md/raw?ref=master to .../templates/issue/...)

On both point, I need some inputs to progress.

A new issue is the integration of these templates with the already existing instance level templates.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #16814