Add enable_hamilton_in_user_preferences FF

What does this MR do and why?

This MR relates to https://gitlab.com/gitlab-org/gitlab/-/issues/419820. It introduces the FF and the template used for the new Code Suggestions section in Preferences.

Note that a Code Suggestions section is already in Preferences. This one will replace it in due time. See here for more context.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

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

1. In rails console enable the experiment fully

   user = User.find(<your-user-id>)
   Feature.enable(:enable_hamilton_in_user_preferences, user)

2. Visit any group or project member pages such as /-/profile/preferences

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.