Check browser version, blacklisting ie6 end ie7

Add an alert message in case of outdated browser. 
Quite the same as #7597, but implementing a blacklist for ie6 and ie7

Here is a screenshot
![screenshot from 2014-11-18 17 43 28](https://cloud.githubusercontent.com/assets/715065/5091635/6fe32bf0-6f4a-11e4-839e-7d6f2090d9c1.png)

Prefer:

> GitLab may not work properly because you are using an outdated web browser. Please install a [supported web browser](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/install/requirements.md#supported-web-browsers) for a better experience.

Specially click `here` links in web development are frowned upon and `Gitlab` should be `GitLab`.