Change admin mode forms to use `_tab_single` partial

What does this MR do and why?

This MR removes the _tabs_normal partial and uses _tab_single instead because the secondary registration tab is never used. This is because of the following discussion.

It also fixes the following UI issues:

• Makes sure the buttons are using btn-confirm rather than btn-success
• Adds padding back to the tab contents

Screenshots or screen recordings

Page	Before	After
Admin mode
2FA for admin mode

How to set up and validate locally

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

1. Login as an Admin and go to [GDK_HOST]/admin/application_settings/general#js-signin-settings
2. Enable admin mode and enforce two-factor authentication
3. Set up 2FA for your admin account
4. You will then be redirected
5. Check the Enter admin mode tab is shown correctly on the password view
6. Login with your password
7. Check the Enter admin mode tab is shown correctly on the 2FA view

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 #230778 (closed)