Set email confirmation setting from soft email confirmation feature flag

What does this MR do and why?

What

The :soft_email_confirmation feature flag is being removed and is being converted to be part of application settings as an additional value in the email_confirmation_setting enumerable

Why

As part of an experiment that was deemed experimentinvalidated we still want to keep the core functionality and move it into an application setting.

Context

Email confirmation settings were added as an application setting in !101564 (merged) and soft was excluded in the UI as an option until this MR.

Screenshots or screen recordings

Before	After	After Login (with MR Changes)

How to set up and validate locally

1. Under Admin Area > Settings > General > Sign-up restrictions

2. Uncheck Require admin approval for new sign-ups if checked

   (or via the rails console (gdk rails c) using: ApplicationSetting.last.update(require_admin_approval_after_user_signup: false)

3. Set Email confirmation settings to Soft

   (or via the rails console (gdk rails c) using: ApplicationSetting.last.update(email_confirmation_setting: 'soft'))

4. Logout and create a new user (via http://gdk.test:3000/users/sign_up)

5. Login with user

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)