WIP: Resolve "Project custom file templates"

What does this MR do?

Add custom file templates with the following architecture:

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

So far I've copied the RepoTemplateFinder from issue and merge request file templates and added a new template using this finder in the new file template finder. It searches all the directories in the folder ".gitlab/personalised_templates" when selected. For now it gets the template via the API using GET /api/v4/projects/19/repository/files/.gitlab/personalised_templates/Documents/blog-post.md/raw?ref=master and I can't figure out how to get a link like the issue templates at .../templates/issue/....

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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 #26199 (moved)