Changed explore area to match more to the style and user-experience of GitLab

Hey,

first of all. We like the new explore area very much and it enhances the user experience for our colleagues a lot.

But after the switch to 7.2. many of our co-workers came to me and said, that it's not fitting really to the overall style of GitLab. The user got accustomed to a navigation below the head panel and to a more defensive and streamlined style.

**So what does this MR do**
This MR changes the navigation from the tab system to the normal sub-navigation system.
Additionally the big prominent text to explain what this area is for changed to a bit more defensive header title.
Therefore, the style is more aligned to the overall style and flow of the tool.

**New look**
![explore_area_revised](https://cloud.githubusercontent.com/assets/6304496/4089572/714bc298-2f6e-11e4-89e3-0b5effa53eb1.jpg)

**Old look**
![explore_area_old](https://cloud.githubusercontent.com/assets/6304496/4089583/c559524c-2f6e-11e4-8a91-1e283209d2a6.jpg)

Each level should be indented 2 spaces instead of 4.