Add email custom text to registration features

What does this MR do and why?

We want to add Email custom text to features available to users who have service ping data sharing enabled.

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

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

1. Testing previous behavior:
   • Make sure you're on a Premium/Ultimate plan GitLab GDK instance
   • Define email custom text (`Admin -> Settings -> Preferences -> Expand Email Section -> Enter Additional text)
   • Try to use the feature - it should be enabled and working according to the documentation
     • Define email custom text (Admin -> Settings -> Preferences -> Expand Email Section -> Enter Additional text)
     • Try to send Forgot password email
2. When registration features are enabled:
   • Make sure you're on a free plan GitLab GDK instance (for example, remove current license or stub #current to return nil)
   • Make sure you have the Registration Features checkbox enabled (Admin -> Settings -> Metrics and profiling -> Usage statistics -> Enable Service Ping & Enable Registration Features)
   • Try to use the feature - it should be enabled and working according to the documentation
     • Define email custom text (Admin -> Settings -> Preferences -> Expand Email Section -> Enter Additional text)
     • Try to send Forgot password email
3. When registration features are disabled:
   • Make sure you're on a free plan GitLab GDK instance
   • Make sure you have the registration features checkbox disabled (Admin -> Settings -> Metrics and profiling -> Usage statistics -> Enable Registration Features)
   • Email custom text should not be available in settings (Admin -> Settings -> Preferences -> Expand Email Section -> Additional text)
   • Email custom text should not be visible in email (Try to send Forgot password email)
4. Make sure that the new text appears on the docs page:
   • run gdk restart gitlab-docs
   • go to <local_gitlab_docs_host>/ee/user/admin_area/settings/usage_statistics.html#registration-features-program and make sure the new section is on the page and the link works

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 #422274 (closed)