Fix preferences link in personal homepage banner

What does this MR do and why?

This fixes the preferences link that gets rendered in the personal homepage's promo banner. It could previously be broken due to being hardcoded client-side. We now build it server-side to ensure it honors the relative_url_root instance setting.

References

• New homepage contains incorrect link to prefenc... (#578217)

Screenshots or screen recordings

How to set up and validate locally

1. Add relative_url_root: '/gitlab' to your gdk.yml.

2. Run gdk reconfigure.

3. Run gdk restart.

4. Navigate to the homepage at https://gdk.test:3443/gitlab.

5. If you had previously dismissed the banner, you can force it to show up again by deleting the associated callouts from the DB, eg:

   rails c
   > Users::Callout.where(feature_name: 'personal_homepage_preferences_banner').delete_all

MR acceptance checklist

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