Move project file links to tab bar.

The sidebar was getting awfully crowded, so I moved the file links to the tab bar alongside Activity and Readme. Unlike Readme, these files are not actually loaded in a tab, but simply link to the relevant blob, because we don't want to unnecessarily load the potentially large changelog or contribution guide on every project page load.

![Screen_Shot_2015-04-06_at_15.04.21](https://gitlab.com/gitlab-org/gitlab-ce/uploads/962e8073ab6dc8797f93ee754eb275ed/Screen_Shot_2015-04-06_at_15.04.21.png)

See merge request !497