Normalize vertical and horizontal space in login pages

What does this MR do and why?

We remove .login-box.

After the restyle of the login page, the .login-box class can be removed. Before the new style, the .login-box container used to have have borders, but not anymore. We decide to remove this div container.

We replaced paddings by margins because margins have the advantage of being collapsible (the larger margin from two neighboring elements wins).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No perceptible changes

How to set up and validate locally

On an incognito window, visit the following pages:

• https://gdk.test:3443/users/sign_in
• https://gdk.test:3443/users/sign_up
• https://gdk.test:3443/-/trial_registrations/new (emulate SaaS)
• https://gdk.test:3443/users/password/edit?reset_password_token=dummy
• https://gdk.test:3443/users/unlock?token=dummy
• https://gdk.test:3443/groups/flightjs/-/saml/sso (SAML enable)
• https://gdk.test:3443/users/sign_up/welcome#login-pane
• https://gdk.test:3443/users/confirmation/new
• https://gdk.test:3443/-/users/terms