Fix clipping on login view

Prior to this change, there's an issue where the login view does not expand to fill the window, leaving a strange buffer space at the bottom:

This MR sets the view to vexpand so that we fill the available space:

Note that this has the side effect of vertically centering the content rather than aligning it to the start. I think it looks good, but it does change the appearance so let me know if we need to change the alignment.