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/14787https://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
0c0ef7dfb6