Add Confidentiality Checkbox to New Test Case Form

As part of &11229 (closed), we will be adding support for creating confidential test cases

While test cases will be migrated to the work items framework mid-next year, this is an interim solution to help enable the Quality team to manage test cases within a single source of truth for ongoing confidential projects.

Proposal

Add checkbox to IssuableForm and update TestCaseCreateRoot to save confidentiality on test case creation

The checkbox should state the following (similar to issues): This test case is confidential and should only be visible to team members with at least Reporter access.

When checked, the test case should become confidential on saving. Group and project members who do not have Reporter access or above should receive a 404 page when attempting to visit the test case link.