Allow users to create confidential issues in private projects

Closes #14787

## What does this MR do?

Allow users to create confidential issues in private projects, and exclude access to them to project members with `Guest` role.

## Are there points in the code the reviewer needs to double check?

The query generated by the `User#authorized_projects` method.

## Why was this MR needed?

Community have been requesting this feature.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/14787

https://gitlab.com/gitlab-org/gitlab-ce/issues/3678

## Screenshots (if relevant)

Not relevant.

## Todo

- [x] Allow users to create confidential issues in private projects
- [x] Project members with `Guest` role should not have access to confidential issues
- [ ] ~~Apply changes in EE + Elasticsearch~~ Will be done in another MR, when this got merged

See merge request !3471