Link mentioned user in markdown only if they are visible to viewer (#21554)
We need to make sure a user can't confirm the existence of a user with
private visibility
* Follow up on #21533
### Before
#### User
![image](https://user-images.githubusercontent.com/20454870/197357580-340911d7-1659-4fc9-a9f6-7ed6bc3476b4.png)
#### Admin
![image](https://user-images.githubusercontent.com/20454870/197357676-a8f0ae63-8f80-4221-a9b5-b6311552910a.png)
### After
#### User
![image](https://user-images.githubusercontent.com/20454870/197357536-05616edb-7821-469d-8e51-6f8cb84c1362.png)
#### Admin
![image](https://user-images.githubusercontent.com/20454870/197357703-071fe984-de79-43aa-a77c-a85b046292a4.png)
Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>