Add changelog, license and contribution guide links to project sidebar.

If the version is known, the "Version: 7.9.0pre" button links to the changelog, otherwise a "View changelog" button is shown.

![Screen_Shot_2015-03-18_at_14.22.29](https://dev.gitlab.org/gitlab/gitlabhq/uploads/747acc018ec9a188098e0f0bc4e91cc3/Screen_Shot_2015-03-18_at_14.22.29.png)

See merge request !1708