Unlink achievement visibility from namespace visibility

What does this MR do and why?

Unlink achievement visibility from namespace visibility

Screenshots or screen recordings

How to set up and validate locally

1. Enable the Achievements feature: Feature.enable(:achievements)
2. Create a private group
3. Create an achievement in that group: https://docs.gitlab.com/ee/user/profile/achievements.html#create-an-achievement
4. Award the achievement to a user: https://docs.gitlab.com/ee/user/profile/achievements.html#award-an-achievement
5. Impersonate (or log in as) a random user which is not a member of the private group
6. Visit the profile of the user which you have awarded the achievement to
7. Verify that the achievement shows up, but doesn't expose the group name/path

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #398634 (closed)