Sign-in form design updates

[bl-nero]

What happened?

Starting from version 15.2.2, we began updating the sign-in form design. As of version 15.2.5, here are some examples of how it may look like, depending on the Auth Service configuration:

The most important change is that the secondary sign-in methods are now displayed right below the default one — with exception of a local username and password login, which still leads to a separate form if it's configured as a secondary method.

Between versions 15.2.2 and 15.2.4, the dialog was also significantly wider, and the SSO buttons were put in a two-column layout. We reverted this part of changes in version 15.2.5, going back to a more narrow design.

Why did we change it?

The main motivation behind this change is promoting passwordless authentication. Previously, even if passwordless authentication was enabled in the Auth Service configuration, it had to be configured as a default method to be visible right away in the sign-in form; to access it, the user needed to click the "Other sign-in options" link. After this change, the passwordless method is always displayed on the first screen if available.

[webvictim]

Some related instructions to disable local or passwordless authentication if needed: #40996