Improve tabbing usability for sign in page

What does this MR do?

Rearranged the html elements so that the sign in button gets tabbed first before the other elements and used CSS to rearrange the visual layout.

I did not use tabindex because it does not play well with screen readers and is not recommended from an accessibility standpoint.

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

• Need to make sure that there aren't any side effects to my code change. .new_user seemed to be automatically added through form_for() and I'm not sure if there will be other cases where .new_user is used where there is no sign in page with forget your password.

Why was this MR needed?

Improves the tabbing order

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23279 (closed)