Fix empty project UI

In an empty project, the 'project is empty' light well was mashed up against the project panel. The light well have 16px margin so there was some white space along the edges that looked funny. This MR makes the 'project is empty' text block a `gray-content-area` which fits nicely with the rest of the UI.

**Current:**

![Screen_Shot_2015-09-14_at_11.20.47_AM](https://gitlab.com/dblessing/gitlab-ce/uploads/07b39641f602e01440dd5558a0d80237/Screen_Shot_2015-09-14_at_11.20.47_AM.png)

**As proposed:**

![Screen_Shot_2015-09-14_at_11.20.29_AM](https://gitlab.com/dblessing/gitlab-ce/uploads/1c3b1d73584c68a88cd1b0fcf3d6ea5b/Screen_Shot_2015-09-14_at_11.20.29_AM.png)

See merge request !1291