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

#### Admin

### After
#### User

#### Admin

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>