`Confidential:` issue filter

Problem to solve

A user should be able to filter issues based on whether the issue is confidential or not.

Further details

Currently, issues can not be filtered by whether they are confidential or not.

Proposal

Implement a filter for confidential:true similar to the other issue filters. confidential:true and confidential:false should be available. present and yes should be accepted as synonyms for true

What does success look like, and how can we measure that?

Usage data and traffic will show use of the confidential filter.

Links / references

This is related to #17326 (closed), which focused on search visibility and #29784 (moved) which has a scope much larger then this MVC.

Design