Can't create epic from within confidential epic tree

Summary

Attempting to 'Add a New Epic' within the epic tree of a confidential epic fails with a generic error.

Steps to reproduce

1. Create a confidential epic;
2. Visit the epic and click 'Add' in the epic tree;
3. Select 'Add a New Epic';
4. Enter a title and click 'Create Epic', and
5. Observe failure to create and a generic error message.

Example Project

Any confidential epic in any group will produce this behaviour.

What is the current bug behavior?

No epic created, generic error message shown.

What is the expected correct behavior?

Confidential sub-epic created and shown within the epic tree.

Relevant logs and/or screenshots

Possible fixes

This functionality uses the REST API and probably fails because it tries to create a public epic, which is invalid as its parent is confidential.

POST requests to /api/v4/groups/:group/epics/:iid/epics should default to confidential when the parent epic is confidential.