Permit Guests to mark issues as confidential

Release notes

Guest roles are now able to mark authored issues as confidential after creation.

Problem to solve

Guests can create confidential issues when marking them during creation, but if they've forgotten to do it (or decide to do it later) an edit option is unavailable

This causes them to rely on other users to mark their issues as confidential.

Proposal

The lack of this ability is because :admin_issue permission is not enabled for them (requires Reporter role at minimum): https://gitlab.com/gitlab-org/gitlab/blob/master/app/policies/project_policy.rb