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

Previously, the padding and margins on cells were all over the place. There was also extra vertical padding that took up too much vertical space, even for simple cells.  Here is a summary of the changes.

* Introduce a `@code_padding` variable that is `0.4em`. This is the padding we use in the CodeMirror editor and has to be used in various places to get a consistent design.
* Removed 2px margin between cells.
* Moved some output related less code from `cell.less` to `outputarea.less`.
* Removed the 5px top margin from the output wrapper. This was causing the output area to occupy vertical space even when empty.
* All `output_subarea` instances now have the `@code_padding` on the top and sides. This means that there is a consistent 0.4em spacing between different outputs in the same cell. This is less that the 10px spacing that was there before.

The overall visual look is slightly tighter vertically.

Here is a screenshot before:

![before](https://f.cloud.github.com/assets/27600/1597360/50143a7e-530f-11e3-9785-9bbbd58d4011.png)

And after:

![after](https://f.cloud.github.com/assets/27600/1597361/5411d5d2-530f-11e3-81ea-45dccf578b22.png)