Bug 28786: (follow-up) Improve style of 2FA code input

Bug 28786: (follow-up) Improve style of 2FA code input

This patch cleans up login.css a little bit so that the same style rules
which apply to the login screen apply to the 2FA input form as well.

The patch also changes the "Log out" link on the 2FA form to a "Cancel"
link alongside the "Verify" button.

To test, apply the patch and start the process of logging in to the
staff client using an account with 2FA enabled.

On both the login form and 2FA code views, confirm that everything is
styled consistently.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>