Making the cell margin 0 all around.

Also moves some css/less code to the outputarea.less.
Adjusting padding of output subareas and adding @code_padding.
Tabs to spaces in variables.less.
More tabs to spaces on less files.
Cleaning up indentation and spacing of less files touched.
Merge pull request #4576 from ellisonbg/cell-spacing

Tighten up the vertical spacing on cells and make the padding of cells more consistent