Add form-group around the checkbox on sign in page

What does this MR do and why?

I'm trying to fix the check/radio button margins on Pajamas in this MR gitlab-ui!3576. And it will affect the login page. Because the checkbox Remember me should have a form-group wrapper. So I added it.

Without the form-group after checkbox fix, it would look like this:

Screenshots or screen recordings

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

Now	After this MR	After checkbox fix

How to set up and validate locally

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

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.