Fix Spacing Above Print Statements in Document Mode

[aarondr77]

[aarondr77]

Empty Cells should take up no space in Document Mode.

[aarondr77]

Headers should have a little bit more space above them. This needs to be applied consistently to the header elements so that if you have

Markdown Cell:
First H2
First Text
Second H2
SecondText

Markdown Cell:
Third H2
Third Text

The spacing between the Second Text and the Third H2 should be identical to the spacing between the First Text and Second H2. That means the markdown cell itself should not have top margin.