Draft: Add soft_email_confirmation to an application setting

What does this MR do and why?

This adds soft_email_confirmation to the application_settings table and adds the UI to toggle the setting.

This is part of #371638 (closed) (Convert soft_email_confirmation FF to an application setting). The next steps after this MR is to create a post_migrate migration to set the application setting based on the feature flag setting.

Screenshots or screen recordings

How to set up and validate locally

1. Go to the Admin > General > Sign-up restrictions
2. Toggle the checkbox
3. Check the application_settings table for an updated value

   1. via rails console (gdk rails c):

       ApplicationSetting.current.soft_email_confirmation

   2. via sql query:

      select soft_email_confirmation from application_settings;

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 #371638 (closed)