Add ability to disable social feature (following)

What does this MR do and why?

Recent request from one of GitLab users identified the need of switching off the social feature (following). This MR let user disable the social feature throgh UI.

Disabling the social feature has the following consequences:

• noone can follow this user
• this user cannot follow anyone else
• all the current relations are destroyed.

Feature is behind a disabling_following feature flag.

Screenshots or screen recordings

https://www.loom.com/share/af5f3fa9f27944d0822092b4d438520c

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the Feature flag

   Feature.enable(:disabling_following)

2. Visit any user's profile. See if the follow button is present and click to follow.
3. Enter profile's preference: http://127.0.0.1:3000/-/profile/preferences
4. Toggle the 'disable following feature' and save.
5. See if the relations were deleted and if you can follow others.

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.

Relates to #325558 (closed)