Add missing classes to omniauth remember-me checkbox and add correct font-size

What does this MR do?

This is a fix for the issue reported at https://gitlab.com/gitlab-org/gitlab-ce/issues/37551

Are there points in the code the reviewer needs to double check?

I made the markup consistent with the top remember me checkbox as they looked like generic classes in form.scss Can add them as distinct classes if that isn't the case.

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/37551