Twenty Nineteen: Reduce excess space at the bottom of the page

[kjellr]

As noted by @dartiss on the Twenty Nineteen GitHub repository, there's a bit of extra space on the bottom of the page under the following circumstances:

1. You're logged in and can see the "Edit" link
2. You're viewing a single page (not a post)
3. You do not have any widgets in the widget area

That extra spacing doesn't show up for logged-out visitors. This should be cleaned up, but since it happens under such specific criteria, the difficult part of a fix is just ensuring that a fix doesn't remove space in other viewing circumstances.

More background here: ​https://github.com/WordPress/twentynineteen/issues/584

Screenshot:

[sabernhardt]

As part of the ​proposal to retire default themes, Twenty Nineteen might not have any more enhancements soon.

On screens larger than 768 pixels wide, the 3rem bottom margin from .entry-footer plus 3rem top margin from .site-info totals 132 pixels at the standard font size.

@xkon ​suggested on the original issue that the .entry-footer bottom margin could be reduced to 1rem for logged-in users.

More than four years after the ticket was opened, I suggest leaving the margins unchanged.