LPS-47843 Device preview has two scroll bars

Hey @pat270,

Here is an update for https://issues.liferay.com/browse/LPS-47843.

The overflow: auto set on .modal-body causes all dialogs to have scroll bars appearing even when there's no overflow. And this causes the device preview to have two scroll bars showing up. That's why I'm removing that rule in the application.css file. I also took the chance to make the sign in modal bigger so that all content can be visible without overflow.

Please let me know if there's any issues. Thanks!