Draft: Sign in UI refactor

What does this MR do and why?

The aim is to make the Omniauth buttons more prominent.

Partially addresses #386487 (closed)

Screenshots or screen recordings

General sign in page

Admin page

The placement of the button on the top would be similar to the sign up form in the trial registration where the external auth buttons are at the top:

https://gdk.test:3443/-/trial_registrations/new

But different from the normal sign up form:

https://gdk.test:3443/users/sign_up

How to set up and validate locally

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.