Issue and MR descriptions have white background.

## White background for rendered markdown issue description

- every place that renders user input markdown should have a single background color so users can easily tell it is rendered markdown

- the gray background is used everywhere for titles, not for main content. We already have the vertical border to visually unite the metadata with the description.

- it is easier to distinguish metadata from description, since they were side by side and had the same color before.

- same as GitHub

New:

![screenshot from 2014-02-23 12 29 55 new](https://f.cloud.github.com/assets/1429315/2240197/67d298fa-9c7e-11e3-81ab-bd5f01f35bba.png)

Old:

![screenshot from 2014-02-23 12 15 42 old](https://f.cloud.github.com/assets/1429315/2240198/6ed01786-9c7e-11e3-8027-811e6e7b8b73.png)

Analogous for MRs and milestones.

**Implementation Notes**

If the background is white, it becomes apparent that there is no `padding-top` and no `border-top`, so some was added to the description.